* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --bg-primary: #fafafa;
    --bg-secondary: #f5f5f5;
    --bg-elevated: #ffffff;
    --text-primary: #1a1a1a;
    --text-secondary: #6b6b6b;
    --text-tertiary: #9ca3af;
    --accent: #1a1a1a;
    --accent-text: #ffffff;
    --border: #e5e5e5;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -2px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -4px rgba(0, 0, 0, 0.1);
    --transition: 300ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-slow: 500ms cubic-bezier(0.4, 0, 0.2, 1);
}

@media (prefers-color-scheme: dark) {
    :root {
        --bg-primary: #1a1a1a;
        --bg-secondary: #0f0f0f;
        --bg-elevated: #1f1f1f;
        --text-primary: #f5f5f7;
        --text-secondary: #b0b0b5;
        --text-tertiary: #7a7a80;
        --accent: #f5f5f7;
        --accent-text: #111111;
        --border: #303030;
        --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.4), 0 2px 4px -2px rgba(0, 0, 0, 0.4);
        --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.5), 0 4px 6px -4px rgba(0, 0, 0, 0.5);
    }
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "SF Pro Display", "SF Pro Text", sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
    -webkit-font-smoothing: antialiased;
    animation: fadeIn 0.6s ease-out;
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

.container {
    max-width: 720px;
    margin: 0 auto;
    padding: 48px 24px;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

.back-link {
    display: inline-flex;
    align-items: center;
    font-size: 14px;
    color: var(--text-secondary);
    text-decoration: none;
    margin-bottom: 48px;
    transition: color 150ms;
    width: fit-content;
}

.back-link:hover {
    color: var(--text-primary);
}

.header {
    text-align: left;
    margin-bottom: 56px;
    animation: slideUp 0.6s ease-out 0.1s both;
}

@keyframes slideUp {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.header h1 {
    font-size: 36px;
    font-weight: 600;
    letter-spacing: -0.025em;
    margin-bottom: 8px;
    color: var(--text-primary);
}

.subtitle {
    font-size: 16px;
    color: var(--text-secondary);
}

.donation-section {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 32px;
}

.amount-selector {
    animation: slideUp 0.6s ease-out 0.2s both;
}

.hint {
    font-size: 13px;
    color: var(--text-tertiary);
    margin-bottom: 16px;
    text-align: left;
}

.amount-buttons {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
}

@media (min-width: 640px) {
    .amount-buttons {
        grid-template-columns: repeat(4, 1fr);
    }
}

.amount-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 4px;
    padding: 20px 16px;
    background: var(--bg-elevated);
    border: 1px solid var(--border);
    border-radius: 16px;
    cursor: pointer;
    transition: all var(--transition);
    color: var(--text-primary);
}

.amount-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow);
}

.amount-btn:active {
    transform: translateY(0);
}

.amount-btn.selected {
    background: var(--accent);
    border-color: var(--accent);
    color: var(--accent-text);
    box-shadow: var(--shadow-lg);
    transform: scale(1.02);
}

.amount-btn.selected .label {
    color: var(--accent-text);
    opacity: 0.82;
}

.amount {
    font-size: 22px;
    font-weight: 600;
}

.label {
    font-size: 13px;
    color: var(--text-tertiary);
    transition: color var(--transition);
}

.qr-section {
    opacity: 0;
    transform: translateY(20px);
    pointer-events: none;
    transition: opacity var(--transition-slow), transform var(--transition-slow);
}

.qr-section.visible {
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

.qr-wrapper {
    display: flex;
    justify-content: center;
    gap: 16px;
}

.qr-wrapper img {
    width: 200px;
    height: auto;
    opacity: 0;
    transition: opacity var(--transition), transform var(--transition);
    cursor: pointer;
    display: block;
}

@media (min-width: 640px) {
    .qr-wrapper img {
        width: 260px;
    }
}

.qr-wrapper img.loaded {
    opacity: 1;
}

.qr-wrapper img:hover {
    transform: scale(1.02);
}

.qr-hint {
    text-align: center;
    font-size: 13px;
    color: var(--text-tertiary);
    margin-top: 20px;
}

@media (max-width: 639px) {
    .container {
        padding: 32px 20px;
    }

    .header h1 {
        font-size: 28px;
    }

    .back-link {
        margin-bottom: 32px;
    }

    .header {
        margin-bottom: 40px;
    }

    .amount-btn {
        padding: 16px 12px;
    }

    .amount {
        font-size: 20px;
    }

    .qr-wrapper img {
        width: 160px;
    }
}

@media (prefers-reduced-motion: reduce) {
    *, *::before, *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }
}