/* ==========================================================================
   Universal Calculator v3.0 - Minimalist & Responsive Architecture
   ========================================================================== */

:root {
    --bg-color: #f4f4f9;
    --calc-bg: #ffffff;
    --text-main: #1e293b;
    --text-muted: #94a3b8;
    --accent: #6366f1;
    --accent-hover: #4f46e5;
    --key-bg: #f1f5f9;
    --key-text: #334155;
    --key-action-bg: #fee2e2;
    --key-action-text: #ef4444;
    --shadow: 0 10px 30px -10px rgba(0, 0, 0, 0.1);
}

[data-theme="dark"] {
    --bg-color: #0f172a;
    --calc-bg: #1e293b;
    --text-main: #f8fafc;
    --text-muted: #64748b;
    --accent: #818cf8;
    --accent-hover: #6366f1;
    --key-bg: #334155;
    --key-text: #f1f5f9;
    --key-action-bg: #7f1d1d;
    --key-action-text: #fca5a5;
    --shadow: 0 10px 40px -10px rgba(0, 0, 0, 0.5);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    -webkit-tap-highlight-color: transparent;
}

body {
    font-family: 'Inter', 'Cairo', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-main);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    transition: background-color 0.4s ease, color 0.4s ease;
    overflow: hidden;
    padding: 20px;
}

/* Container Layout (Flexbox) */
.app-container {
    width: 100%;
    max-width: 400px;
    background-color: var(--calc-bg);
    border-radius: 24px;
    box-shadow: var(--shadow);
    display: flex;
    flex-direction: column;
    overflow: hidden;
    height: 95vh;
    max-height: 800px;
    transition: all 0.4s ease;
}

/* Header */
.app-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 16px 24px;
    border-bottom: 1px solid rgba(0,0,0,0.05);
}

.lang-select, .btn-toggle {
    background: var(--key-bg);
    color: var(--key-text);
    border: none;
    padding: 8px 12px;
    border-radius: 8px;
    font-weight: 600;
    cursor: pointer;
    font-size: 0.85rem;
    transition: 0.2s;
}

.lang-select:hover, .btn-toggle:hover {
    filter: brightness(0.95);
}

.header-right { display: flex; gap: 8px; }

/* Display Screen */
.display-screen {
    flex: 1;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    align-items: flex-end;
    padding: 24px;
    min-height: 120px;
    word-wrap: break-word;
    word-break: break-all;
}

.history-display {
    font-size: 1rem;
    color: var(--text-muted);
    margin-bottom: 8px;
    min-height: 1.2rem;
    text-align: right;
}

.current-input {
    font-size: clamp(2rem, 8vw, 3.5rem);
    font-weight: 300;
    letter-spacing: -1px;
    text-align: right;
}

/* Keypad Grid (CSS Grid) */
.keypad-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 8px;
    padding: 16px;
    background: rgba(0,0,0,0.02);
}

.key-btn {
    border: none;
    background-color: var(--key-bg);
    color: var(--key-text);
    padding: 18px 0;
    border-radius: 16px;
    font-size: 1.2rem;
    font-weight: 600;
    cursor: pointer;
    transition: transform 0.1s, background-color 0.2s;
    display: flex;
    justify-content: center;
    align-items: center;
    user-select: none;
}

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

.key-btn[data-type="action"] {
    background-color: var(--key-action-bg);
    color: var(--key-action-text);
}

.key-btn[data-type="op"] {
    background-color: var(--accent);
    color: white;
}

.key-btn[data-type="op"]:active {
    background-color: var(--accent-hover);
}

.key-btn.span-2 { grid-column: span 2; }
.key-btn.span-2.text-left { justify-content: flex-start; padding-left: 24px; }

/* Footer */
.app-footer {
    text-align: center;
    padding: 8px;
    font-size: 0.7rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 2px;
}

/* Modal */
.modal-overlay {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    background: rgba(0,0,0,0.7);
    display: none;
    justify-content: center;
    align-items: center;
    z-index: 100;
    backdrop-filter: blur(5px);
}

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

.modal-content {
    background: var(--calc-bg);
    padding: 40px;
    border-radius: 24px;
    max-width: 350px;
    text-align: center;
    box-shadow: 0 20px 50px rgba(0,0,0,0.3);
    animation: popIn 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

@keyframes popIn {
    from { transform: scale(0.8); opacity: 0; }
    to { transform: scale(1); opacity: 1; }
}

.modal-content h2 { margin-bottom: 12px; }
.modal-content p { color: var(--text-muted); margin-bottom: 24px; line-height: 1.5; }

.btn-primary {
    background: var(--accent);
    color: white;
    border: none;
    padding: 12px 32px;
    border-radius: 12px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: 0.2s;
    width: 100%;
}

.btn-primary:hover { background: var(--accent-hover); }

/* Mobile Adjustments (Responsive-first) */
@media (max-width: 380px) {
    .app-container { height: 100vh; max-height: none; border-radius: 0; }
    body { padding: 0; }
}