/* ===== CSS Custom Properties (Theming) ===== */
:root {
    --bg-primary: #f8f9fa;
    --bg-secondary: #ffffff;
    --bg-tertiary: #e9ecef;
    --text-primary: #212529;
    --text-secondary: #6c757d;
    --text-muted: #adb5bd;

    --grid-border-thick: #212529;
    --grid-border-thin: #ced4da;
    --grid-bg: #ffffff;

    --cell-selected: #bbdefb;
    --cell-highlighted: #e3f2fd;
    --cell-same-number: #c8e6c9;
    --cell-error-bg: #ffcdd2;
    --cell-error-text: #c62828;
    --cell-given-text: #212529;
    --cell-user-text: #1565c0;
    --cell-hint-text: #2e7d32;
    --cell-notes-text: #6c757d;

    --accent: #1976d2;
    --accent-hover: #1565c0;
    --accent-light: #e3f2fd;
    --success: #2e7d32;
    --danger: #c62828;
    --warning: #f57f17;

    --btn-bg: #ffffff;
    --btn-hover: #f1f3f5;
    --btn-active: #e9ecef;
    --btn-border: #dee2e6;

    --modal-bg: #ffffff;
    --modal-overlay: rgba(0, 0, 0, 0.5);

    --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
    --shadow-md: 0 4px 12px rgba(0,0,0,0.1);
    --shadow-lg: 0 8px 24px rgba(0,0,0,0.15);

    --radius-sm: 6px;
    --radius-md: 10px;
    --radius-lg: 16px;

    --transition-fast: 0.15s ease;
    --transition-normal: 0.25s ease;
}

[data-theme="dark"] {
    --bg-primary: #121212;
    --bg-secondary: #1e1e1e;
    --bg-tertiary: #2a2a2a;
    --text-primary: #e0e0e0;
    --text-secondary: #9e9e9e;
    --text-muted: #616161;

    --grid-border-thick: #e0e0e0;
    --grid-border-thin: #424242;
    --grid-bg: #1e1e1e;

    --cell-selected: #1a237e;
    --cell-highlighted: #1a1a3e;
    --cell-same-number: #1b5e20;
    --cell-error-bg: #4a1414;
    --cell-error-text: #ef5350;
    --cell-given-text: #e0e0e0;
    --cell-user-text: #64b5f6;
    --cell-hint-text: #66bb6a;
    --cell-notes-text: #9e9e9e;

    --accent: #42a5f5;
    --accent-hover: #64b5f6;
    --accent-light: #1a237e;
    --success: #66bb6a;
    --danger: #ef5350;
    --warning: #ffb74d;

    --btn-bg: #2a2a2a;
    --btn-hover: #333333;
    --btn-active: #404040;
    --btn-border: #424242;

    --modal-bg: #1e1e1e;
    --modal-overlay: rgba(0, 0, 0, 0.7);

    --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
    --shadow-md: 0 4px 12px rgba(0,0,0,0.4);
    --shadow-lg: 0 8px 24px rgba(0,0,0,0.5);
}

/* ===== Reset & Base ===== */
*, *::before, *::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    -webkit-text-size-adjust: 100%;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background-color: var(--bg-primary);
    color: var(--text-primary);
    min-height: 100dvh;
    display: flex;
    justify-content: center;
    transition: background-color var(--transition-normal), color var(--transition-normal);
    -webkit-tap-highlight-color: transparent;
    user-select: none;
}

/* ===== App Layout ===== */
.app {
    width: 100%;
    max-width: 500px;
    padding: 0.75rem;
    display: flex;
    flex-direction: column;
    min-height: 100dvh;
}

.app-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.5rem 0;
    margin-bottom: 0.5rem;
}

.app-title {
    font-size: 1.5rem;
    font-weight: 700;
    letter-spacing: -0.02em;
}

.header-actions {
    display: flex;
    align-items: center;
    gap: 0.25rem;
}

.icon-btn {
    background: none;
    border: none;
    cursor: pointer;
    color: var(--text-secondary);
    padding: 0.5rem;
    border-radius: var(--radius-sm);
    transition: color var(--transition-fast), background-color var(--transition-fast);
    display: flex;
    align-items: center;
}

.icon-btn:hover {
    color: var(--text-primary);
    background-color: var(--btn-hover);
}

.icon {
    width: 22px;
    height: 22px;
}

[data-theme="light"] .icon-moon { display: none; }
[data-theme="dark"] .icon-sun { display: none; }

/* ===== Game Container ===== */
.game-container {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.75rem;
    flex: 1;
}

/* ===== Info Bar ===== */
.info-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    width: 100%;
    max-width: 450px;
    padding: 0 0.25rem;
}

.difficulty-badge {
    font-size: 0.8rem;
    font-weight: 600;
    padding: 0.25rem 0.75rem;
    border-radius: 100px;
    background-color: var(--accent-light);
    color: var(--accent);
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.mistakes {
    display: flex;
    align-items: center;
    gap: 0.35rem;
    font-size: 0.85rem;
    color: var(--text-secondary);
}

.icon-sm {
    width: 16px;
    height: 16px;
}

.timer {
    font-size: 0.95rem;
    font-weight: 600;
    font-variant-numeric: tabular-nums;
    color: var(--text-secondary);
}

/* ===== Grid ===== */
.grid-wrapper {
    position: relative;
    width: min(90vw, 450px);
    aspect-ratio: 1;
}

.grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    width: 100%;
    height: 100%;
    border: 2.5px solid var(--grid-border-thick);
    border-radius: var(--radius-sm);
    overflow: hidden;
    background-color: var(--grid-bg);
    box-shadow: var(--shadow-md);
}

.cell {
    display: flex;
    align-items: center;
    justify-content: center;
    border: 0.5px solid var(--grid-border-thin);
    cursor: pointer;
    position: relative;
    font-size: clamp(1.1rem, 4.5vw, 1.6rem);
    font-weight: 600;
    transition: background-color var(--transition-fast);
    background-color: var(--grid-bg);
}

/* 3x3 box borders */
.cell[data-col="2"],
.cell[data-col="5"] {
    border-right: 2px solid var(--grid-border-thick);
}
.cell[data-col="3"],
.cell[data-col="6"] {
    border-left: none;
}
.cell[data-row="2"],
.cell[data-row="5"] {
    border-bottom: 2px solid var(--grid-border-thick);
}
.cell[data-row="3"],
.cell[data-row="6"] {
    border-top: none;
}

/* Cell states */
.cell--given {
    color: var(--cell-given-text);
    font-weight: 700;
}

.cell--user {
    color: var(--cell-user-text);
}

.cell--hint {
    color: var(--cell-hint-text);
}

.cell--selected {
    background-color: var(--cell-selected) !important;
}

.cell--highlighted {
    background-color: var(--cell-highlighted);
}

.cell--same-number {
    background-color: var(--cell-same-number);
}

.cell--error {
    color: var(--cell-error-text) !important;
    background-color: var(--cell-error-bg) !important;
}

/* Cell content */
.cell-value {
    pointer-events: none;
    line-height: 1;
}

.cell-notes {
    display: none;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    padding: 1px;
    pointer-events: none;
}

.cell-notes.visible {
    display: grid;
}

.cell-notes span {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: clamp(0.45rem, 1.8vw, 0.6rem);
    font-weight: 500;
    color: var(--cell-notes-text);
    line-height: 1;
}

/* ===== Generating Overlay ===== */
.generating-overlay {
    position: absolute;
    inset: 0;
    display: none;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    gap: 0.75rem;
    background-color: var(--modal-overlay);
    border-radius: var(--radius-sm);
    color: #fff;
    font-weight: 500;
    z-index: 5;
}

.generating-overlay.visible {
    display: flex;
}

.spinner {
    width: 32px;
    height: 32px;
    border: 3px solid rgba(255,255,255,0.3);
    border-top-color: #fff;
    border-radius: 50%;
    animation: spin 0.8s linear infinite;
}

@keyframes spin {
    to { transform: rotate(360deg); }
}

/* ===== Controls ===== */
.controls {
    width: 100%;
    max-width: 450px;
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
}

/* Number Pad */
.number-pad {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 0.3rem;
}

.num-btn {
    position: relative;
    aspect-ratio: 1;
    border: 1.5px solid var(--btn-border);
    border-radius: var(--radius-sm);
    background-color: var(--btn-bg);
    color: var(--accent);
    font-family: inherit;
    font-size: clamp(1.1rem, 4vw, 1.4rem);
    font-weight: 700;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all var(--transition-fast);
    -webkit-tap-highlight-color: transparent;
}

.num-btn:hover {
    background-color: var(--btn-hover);
    border-color: var(--accent);
}

.num-btn:active {
    background-color: var(--accent-light);
    transform: scale(0.95);
}

.num-btn.completed {
    opacity: 0.3;
    pointer-events: none;
}

.num-btn .remaining {
    position: absolute;
    top: 2px;
    right: 3px;
    font-size: 0.5rem;
    font-weight: 500;
    color: var(--text-muted);
}

/* Action Buttons */
.action-buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 0.3rem;
}

.action-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.2rem;
    padding: 0.5rem;
    border: 1.5px solid var(--btn-border);
    border-radius: var(--radius-sm);
    background-color: var(--btn-bg);
    color: var(--text-secondary);
    font-family: inherit;
    font-size: 0.7rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition-fast);
    -webkit-tap-highlight-color: transparent;
}

.action-btn:hover {
    background-color: var(--btn-hover);
    color: var(--text-primary);
}

.action-btn:active {
    background-color: var(--btn-active);
}

.action-btn.active {
    background-color: var(--accent-light);
    border-color: var(--accent);
    color: var(--accent);
}

.icon-action {
    width: 20px;
    height: 20px;
}

/* Bottom Buttons */
.game-bottom-buttons {
    display: flex;
    gap: 0.5rem;
    width: 100%;
    max-width: 450px;
    margin-top: 0.25rem;
}

.new-game-btn {
    flex: 1;
    padding: 0.75rem;
    border: none;
    border-radius: var(--radius-md);
    background-color: var(--accent);
    color: #ffffff;
    font-family: inherit;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: background-color var(--transition-fast), transform var(--transition-fast);
}

.new-game-btn:hover {
    background-color: var(--accent-hover);
}

.new-game-btn:active {
    transform: scale(0.98);
}

.multiplayer-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.4rem;
    padding: 0.75rem 1rem;
    border: 1.5px solid var(--btn-border);
    border-radius: var(--radius-md);
    background-color: var(--btn-bg);
    color: var(--text-primary);
    font-family: inherit;
    font-size: 0.9rem;
    font-weight: 600;
    cursor: pointer;
    transition: all var(--transition-fast);
    white-space: nowrap;
}

.multiplayer-btn:hover {
    border-color: var(--accent);
    background-color: var(--accent-light);
    color: var(--accent);
}

.multiplayer-btn:active {
    transform: scale(0.98);
}

.multiplayer-btn .icon-action {
    width: 18px;
    height: 18px;
}

/* ===== Player List (Multiplayer) ===== */
.player-list {
    display: flex;
    flex-wrap: wrap;
    gap: 0.4rem;
    width: 100%;
    max-width: 450px;
    padding: 0 0.25rem;
}

.player-tag {
    display: flex;
    align-items: center;
    gap: 0.35rem;
    padding: 0.3rem 0.6rem;
    border-radius: 100px;
    background-color: var(--bg-tertiary);
    font-size: 0.78rem;
    font-weight: 500;
    color: var(--text-primary);
}

.player-tag--me {
    background-color: var(--accent-light);
}

.player-dot {
    width: 8px;
    height: 8px;
    border-radius: 50%;
    opacity: 0.4;
    flex-shrink: 0;
}

.player-dot.online {
    opacity: 1;
    box-shadow: 0 0 0 2px rgba(255,255,255,0.3);
}

.player-name {
    max-width: 100px;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.player-count {
    font-size: 0.65rem;
    font-weight: 700;
    color: #fff;
    min-width: 18px;
    height: 18px;
    border-radius: 9px;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0 4px;
}

/* ===== Toast Notifications ===== */
.toast-container {
    position: fixed;
    bottom: 1.5rem;
    left: 50%;
    transform: translateX(-50%);
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.5rem;
    z-index: 200;
    pointer-events: none;
}

.toast {
    background-color: var(--bg-secondary);
    color: var(--text-primary);
    padding: 0.5rem 1rem;
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-lg);
    font-size: 0.85rem;
    font-weight: 500;
    border-left: 3px solid var(--accent);
    opacity: 0;
    transform: translateY(10px);
    transition: opacity 0.3s ease, transform 0.3s ease;
    white-space: nowrap;
}

.toast.visible {
    opacity: 1;
    transform: translateY(0);
}

/* ===== Multiplayer Forms ===== */
.mp-form {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    margin-bottom: 1rem;
}

.mp-input {
    width: 100%;
    padding: 0.7rem 1rem;
    border: 1.5px solid var(--btn-border);
    border-radius: var(--radius-md);
    background-color: var(--btn-bg);
    color: var(--text-primary);
    font-family: inherit;
    font-size: 0.95rem;
    outline: none;
    transition: border-color var(--transition-fast);
}

.mp-input:focus {
    border-color: var(--accent);
}

.mp-input::placeholder {
    color: var(--text-muted);
}

.mp-section-label {
    font-size: 0.85rem;
    color: var(--text-secondary);
    margin-bottom: 0.5rem;
    text-align: center;
}

.mp-share-group {
    display: flex;
    gap: 0.5rem;
    margin-bottom: 0.5rem;
}

.mp-share-input {
    flex: 1;
    min-width: 0;
    font-size: 0.8rem !important;
}

.mp-copy-btn {
    padding: 0.7rem 1rem;
    border: none;
    border-radius: var(--radius-md);
    background-color: var(--accent);
    color: #fff;
    font-family: inherit;
    font-size: 0.85rem;
    font-weight: 600;
    cursor: pointer;
    transition: background-color var(--transition-fast);
    white-space: nowrap;
}

.mp-copy-btn:hover {
    background-color: var(--accent-hover);
}

.mp-share-hint {
    font-size: 0.78rem;
    color: var(--text-muted);
    text-align: center;
}

/* ===== Modals ===== */
.modal-overlay {
    position: fixed;
    inset: 0;
    display: none;
    align-items: center;
    justify-content: center;
    background-color: var(--modal-overlay);
    z-index: 100;
    padding: 1rem;
    opacity: 0;
    transition: opacity var(--transition-normal);
}

.modal-overlay.visible {
    display: flex;
}

.modal-overlay.visible.fade-in {
    opacity: 1;
}

.modal {
    background-color: var(--modal-bg);
    border-radius: var(--radius-lg);
    padding: 2rem;
    width: 100%;
    max-width: 360px;
    box-shadow: var(--shadow-lg);
    text-align: center;
    transform: scale(0.9);
    transition: transform var(--transition-normal);
}

.modal-overlay.visible .modal {
    transform: scale(1);
}

.modal-title {
    font-size: 1.4rem;
    font-weight: 700;
    margin-bottom: 0.25rem;
}

.modal-subtitle {
    font-size: 0.9rem;
    color: var(--text-secondary);
    margin-bottom: 1.25rem;
}

.difficulty-options {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    margin-bottom: 1rem;
}

.difficulty-btn {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.85rem 1rem;
    border: 1.5px solid var(--btn-border);
    border-radius: var(--radius-md);
    background-color: var(--btn-bg);
    cursor: pointer;
    transition: all var(--transition-fast);
    font-family: inherit;
    text-align: left;
}

.difficulty-btn:hover {
    border-color: var(--accent);
    background-color: var(--accent-light);
}

.diff-label {
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-primary);
}

.diff-desc {
    font-size: 0.8rem;
    color: var(--text-secondary);
}

.modal-cancel {
    background: none;
    border: none;
    color: var(--text-secondary);
    font-family: inherit;
    font-size: 0.9rem;
    cursor: pointer;
    padding: 0.5rem 1rem;
    transition: color var(--transition-fast);
}

.modal-cancel:hover {
    color: var(--text-primary);
}

/* Win Modal */
.win-icon {
    font-size: 3rem;
    margin-bottom: 0.5rem;
}

.win-stats {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 0.75rem;
    margin: 1.25rem 0;
    text-align: center;
}

.stat {
    display: flex;
    flex-direction: column;
    gap: 0.15rem;
}

.stat-value {
    font-size: 1.3rem;
    font-weight: 700;
    color: var(--accent);
}

.stat-label {
    font-size: 0.75rem;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

/* ===== Animations ===== */
@keyframes cellPop {
    0% { transform: scale(1); }
    50% { transform: scale(1.15); }
    100% { transform: scale(1); }
}

@keyframes errorShake {
    0%, 100% { transform: translateX(0); }
    20% { transform: translateX(-3px); }
    40% { transform: translateX(3px); }
    60% { transform: translateX(-3px); }
    80% { transform: translateX(3px); }
}

@keyframes hintGlow {
    0% { box-shadow: inset 0 0 0 0 rgba(46, 125, 50, 0.4); }
    50% { box-shadow: inset 0 0 0 3px rgba(46, 125, 50, 0.4); }
    100% { box-shadow: inset 0 0 0 0 rgba(46, 125, 50, 0); }
}

@keyframes winWave {
    0% { transform: scale(1); background-color: var(--grid-bg); }
    50% { transform: scale(1.05); background-color: var(--cell-same-number); }
    100% { transform: scale(1); background-color: var(--grid-bg); }
}

.cell--pop {
    animation: cellPop 0.25s ease;
}

.cell--shake {
    animation: errorShake 0.35s ease;
}

.cell--hint-glow {
    animation: hintGlow 0.8s ease;
}

.cell--win-wave {
    animation: winWave 0.5s ease both;
}

/* ===== Responsive ===== */
@media (max-width: 380px) {
    .app {
        padding: 0.5rem;
    }

    .action-btn {
        padding: 0.4rem;
        font-size: 0.65rem;
    }

    .icon-action {
        width: 18px;
        height: 18px;
    }

    .multiplayer-btn {
        font-size: 0.8rem;
        padding: 0.6rem 0.75rem;
    }
}

@media (min-width: 500px) {
    .app {
        padding: 1.5rem;
    }

    .info-bar {
        padding: 0;
    }
}

@media (min-height: 800px) {
    .game-container {
        gap: 1rem;
    }

    .controls {
        gap: 1rem;
    }
}

/* Focus visible for keyboard navigation */
:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

button:focus:not(:focus-visible) {
    outline: none;
}