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>

plans/demo-mode/CONTEXT.md

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+# Feature Context: Demo Mode
+
+## Current State
+Starting implementation. No changes made yet.
+
+## Key Architecture Notes
+- `EngineRegistry` (class-level dict) holds capture engines, auto-registered in `capture_engines/__init__.py`
+- `AudioEngineRegistry` (class-level dict) holds audio engines, auto-registered in `audio/__init__.py`
+- `LEDDeviceProvider` instances registered via `register_provider()` in `led_client.py`
+- Already has `MockDeviceProvider` + `MockClient` (device type "mock") for testing
+- Config is `pydantic_settings.BaseSettings` in `config.py`, loaded from YAML + env vars
+- Frontend header in `templates/index.html` line 27-31: title + version badge
+- Frontend bundle: `cd server && npm run build` (esbuild)
+- Data stored as JSON in `data/` directory, paths configured via `StorageConfig`
+
+## Temporary Workarounds
+- None yet
+
+## Cross-Phase Dependencies
+- Phase 1 (config flag) is foundational — all other phases depend on `is_demo_mode()`
+- Phase 2 & 3 (engines) can be done independently of each other
+- Phase 4 (seed data) depends on knowing what entities to create, which is informed by phases 2-3
+- Phase 5 (frontend) depends on the system info API field from phase 1
+- Phase 6 (engine resolution) depends on engines existing from phases 2-3
+
+## Implementation Notes
+- Demo mode activated via `WLED_DEMO=true` env var or `demo: true` in YAML config
+- Isolated data directory `data/demo/` keeps demo entities separate from real config
+- Demo engines use `ENGINE_TYPE = "demo"` and are always registered but return `is_available() = True` only in demo mode
+- The existing `MockDeviceProvider`/`MockClient` can be reused or extended for demo device output