Add demo mode: virtual hardware sandbox for testing without real devices

Demo mode provides a complete sandbox environment with:
- Virtual capture engine (radial rainbow test pattern on 3 displays)
- Virtual audio engine (synthetic music-like audio on 2 devices)
- Virtual LED device provider (strip/60, matrix/256, ring/24 LEDs)
- Isolated data directory (data/demo/) with auto-seeded sample entities
- Dedicated config (config/demo_config.yaml) with pre-configured API key
- Frontend indicator (DEMO badge + dismissible banner)
- Engine filtering (only demo engines visible in demo mode)
- Separate entry point: python -m wled_controller.demo (port 8081)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/static/css/layout.css

@@ -129,6 +129,59 @@ h2 {
     50% { opacity: 0.5; }
 }
 
+/* Demo mode badge (header) */
+.demo-badge {
+    font-family: 'Orbitron', sans-serif;
+    font-size: 0.6rem;
+    font-weight: 700;
+    letter-spacing: 0.08em;
+    color: #1a1a1a;
+    background: #ffb300;
+    padding: 2px 10px;
+    border-radius: 10px;
+    text-transform: uppercase;
+    animation: demoPulse 3s ease-in-out infinite;
+    white-space: nowrap;
+}
+
+@keyframes demoPulse {
+    0%, 100% { opacity: 1; }
+    50% { opacity: 0.7; }
+}
+
+/* Demo mode banner (top of page) */
+.demo-banner {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 12px;
+    padding: 8px 16px;
+    background: linear-gradient(90deg, #ff8f00, #ffb300);
+    color: #1a1a1a;
+    font-size: 0.85rem;
+    font-weight: 500;
+    text-align: center;
+    z-index: var(--z-sticky, 100);
+    position: relative;
+}
+
+.demo-banner-dismiss {
+    background: none;
+    border: none;
+    color: #1a1a1a;
+    font-size: 1.2rem;
+    cursor: pointer;
+    padding: 0 4px;
+    opacity: 0.6;
+    transition: opacity 0.2s;
+    line-height: 1;
+    flex-shrink: 0;
+}
+
+.demo-banner-dismiss:hover {
+    opacity: 1;
+}
+
 /* Connection lost overlay */
 .connection-overlay {
     position: fixed;