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/locales/en.json

@@ -4,6 +4,8 @@
   "app.api_docs": "API Documentation",
   "app.connection_lost": "Server unreachable",
   "app.connection_retrying": "Attempting to reconnect…",
+  "demo.badge": "DEMO",
+  "demo.banner": "You're in demo mode — all devices and data are virtual. No real hardware is used.",
   "theme.toggle": "Toggle theme",
   "bg.anim.toggle": "Toggle ambient background",
   "accent.title": "Accent color",