:root {
    --bg-color: #f9fafb;
    --card-bg: #ffffff;
    --text-main: #111827;
    --text-muted: #6b7280;
    --border-color: #e5e7eb;
    --primary: #000000;
    --primary-hover: #374151;
    --error: #ef4444;
    --error-bg: #fef2f2;
    --focus-ring: rgba(0, 0, 0, 0.1);
}

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

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    background-color: var(--bg-color);
    color: var(--text-main);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    padding: 1rem;
    -webkit-font-smoothing: antialiased;
}

.container {
    background: var(--card-bg);
    max-width: 480px;
    width: 100%;
    border-radius: 24px;
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.05), 0 8px 10px -6px rgba(0, 0, 0, 0.01);
    padding: 2.5rem 2rem;
    text-align: center;
}

header h1 {
    font-size: 1.5rem;
    font-weight: 600;
    margin-bottom: 0.5rem;
    letter-spacing: -0.025em;
}

header p {
    color: var(--text-muted);
    font-size: 0.95rem;
    margin-bottom: 2.5rem;
}

.drop-zone {
    border: 2px dashed var(--border-color);
    border-radius: 16px;
    padding: 3rem 1.5rem;
    position: relative;
    cursor: pointer;
    transition: all 0.2s ease;
    background-color: var(--card-bg);
}

.drop-zone:hover, .drop-zone.dragover {
    border-color: var(--primary);
    background-color: #fafafa;
}

.file-input {
    position: absolute;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    opacity: 0;
    cursor: pointer;
}

.drop-zone-content {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 1rem;
    color: var(--text-muted);
}

.drop-zone-content svg {
    color: var(--text-main);
}

.drop-text {
    font-weight: 500;
    font-size: 1rem;
    color: var(--text-main);
}

.sub-text {
    font-weight: 400;
    font-size: 0.85rem;
    color: var(--text-muted);
}

.file-info {
    display: flex;
    align-items: center;
    justify-content: space-between;
    background-color: var(--bg-color);
    padding: 1rem 1.25rem;
    border-radius: 12px;
    border: 1px solid var(--border-color);
    margin-bottom: 1.5rem;
    font-size: 0.95rem;
    font-weight: 500;
}

.clear-btn {
    background: none;
    border: none;
    color: var(--text-muted);
    font-size: 1.25rem;
    line-height: 1;
    cursor: pointer;
    padding: 0.25rem;
    transition: color 0.2s;
}

.clear-btn:hover {
    color: var(--error);
}

.submit-btn {
    width: 100%;
    background-color: var(--primary);
    color: #ffffff;
    border: none;
    border-radius: 12px;
    padding: 1.1rem;
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 56px; /* Touch target size */
}

.submit-btn:not(:disabled):hover {
    background-color: var(--primary-hover);
    transform: translateY(-1px);
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
}

.submit-btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.submit-btn:focus-visible {
    outline: none;
    box-shadow: 0 0 0 3px var(--focus-ring);
}

.error-message {
    margin-top: 1.5rem;
    padding: 1rem;
    background-color: var(--error-bg);
    color: var(--error);
    border-radius: 8px;
    font-size: 0.9rem;
    font-weight: 500;
}

.hidden {
    display: none !important;
}

/* Spinner Loader */
.loader {
    width: 24px;
    height: 24px;
    border: 3px solid rgba(255,255,255,0.3);
    border-radius: 50%;
    border-top-color: #fff;
    animation: spin 1s ease-in-out infinite;
}

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

/* Mobile adjustments */
@media (max-width: 480px) {
    .container {
        padding: 2rem 1.5rem;
        border-radius: 20px;
        box-shadow: none;
    }
    
    body {
        background-color: var(--card-bg); /* Full white on mobile */
        align-items: flex-start; /* Move up slightly on mobile */
        padding-top: 2rem;
    }
}