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>

CLAUDE.md

@@ -114,9 +114,37 @@ powershell -ExecutionPolicy Bypass -File "c:\Users\Alexei\Documents\wled-screen-
 
 **Do NOT use** `Stop-Process -Name python` (kills unrelated Python processes like VS Code extensions) or bash background `&` jobs (get killed when the shell session ends).
 
-## Default Config & API Key
+## IMPORTANT: Server Startup Commands
 
-The server configuration is in `/server/config/default_config.yaml`. The default API key for development is `development-key-change-in-production` (label: `dev`). The server runs on port **8080** by default.
+There are two server modes with separate configs, ports, and data directories:
+
+| Mode | Command | Config | Port | API Key | Data |
+|------|---------|--------|------|---------|------|
+| **Real** | `python -m wled_controller.main` | `config/default_config.yaml` | 8080 | `development-key-change-in-production` | `data/` |
+| **Demo** | `python -m wled_controller.demo` | `config/demo_config.yaml` | 8081 | `demo` | `data/demo/` |
+
+Both can run simultaneously on different ports.
+
+### Restarting after code changes
+
+- **Real server**: Use the PowerShell restart script (it only targets the real server process):
+
+  ```bash
+  powershell -ExecutionPolicy Bypass -File "c:\Users\Alexei\Documents\wled-screen-controller\server\restart.ps1"
+  ```
+
+- **Demo server**: Find and kill the process on port 8081, then restart:
+
+  ```bash
+  # Find PID
+  powershell -Command "netstat -ano | Select-String ':8081.*LISTEN'"
+  # Kill it
+  powershell -Command "Stop-Process -Id <PID> -Force"
+  # Restart
+  cd server && python -m wled_controller.demo
+  ```
+
+**Do NOT use** `Stop-Process -Name python` — it kills unrelated Python processes (VS Code extensions, etc.).
 
 ## Project Structure
 
@@ -146,6 +174,25 @@ Use `TODO.md` in the project root as the primary task tracker. **Do NOT use the
 
 **Use context7 MCP tools for library/framework documentation lookups.** When you need to check API signatures, usage patterns, or current behavior of external libraries (e.g., FastAPI, OpenCV, Pydantic, yt-dlp), use `mcp__plugin_context7_context7__resolve-library-id` to find the library, then `mcp__plugin_context7_context7__query-docs` to fetch up-to-date docs. This avoids relying on potentially outdated training data.
 
+## IMPORTANT: Demo Mode Awareness
+
+**When adding new entity types, engines, device providers, or stores — keep demo mode in sync:**
+
+1. **New entity stores**: Add the store's file path to `StorageConfig` in `config.py` — the `model_post_init()` auto-rewrites `data/` → `data/demo/` paths when demo is active.
+2. **New capture engines**: If a new engine is added, verify demo mode filtering still works (demo engines use `is_demo_mode()` gate in `is_available()`).
+3. **New audio engines**: Same as capture engines — `is_available()` must respect `is_demo_mode()`.
+4. **New device providers**: If discovery is added, gate it with `is_demo_mode()` like `DemoDeviceProvider.discover()`.
+5. **New seed data**: When adding new entity types that should appear in demo mode, update `server/src/wled_controller/core/demo_seed.py` to include sample entities.
+6. **Frontend indicators**: Demo mode state is exposed via `GET /api/v1/version` → `demo_mode: bool`. Frontend stores it as `demoMode` in app state and sets `document.body.dataset.demo = 'true'`.
+7. **Backup/Restore**: If new stores are added to `STORE_MAP` in `system.py`, they automatically work in demo mode since the data directory is already isolated.
+
+**Key files:**
+
+- Config flag: `server/src/wled_controller/config.py` → `Config.demo`, `is_demo_mode()`
+- Demo engines: `core/capture_engines/demo_engine.py`, `core/audio/demo_engine.py`
+- Demo devices: `core/devices/demo_provider.py`
+- Seed data: `core/demo_seed.py`
+
 ## General Guidelines
 
 - Always test changes before marking as complete