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/phase-1-config-flag.md

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+# Phase 1: Demo Mode Config & Flag
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** backend
+
+## Objective
+Add a `demo` boolean flag to the application configuration and expose it to the frontend via the system info API. When demo mode is active, the server uses an isolated data directory so demo entities don't pollute real user data.
+
+## Tasks
+
+- [ ] Task 1: Add `demo: bool = False` field to `Config` class in `config.py`
+- [ ] Task 2: Add a module-level helper `is_demo_mode() -> bool` in `config.py` for easy import
+- [ ] Task 3: Modify `StorageConfig` path resolution: when `demo=True`, prefix all storage paths with `data/demo/` instead of `data/`
+- [ ] Task 4: Expose `demo_mode: bool` in the existing `GET /api/v1/system/info` endpoint response
+- [ ] Task 5: Add `WLED_DEMO=true` env var support (already handled by pydantic-settings env prefix `WLED_`)
+
+## Files to Modify/Create
+- `server/src/wled_controller/config.py` — Add `demo` field, `is_demo_mode()` helper, storage path override
+- `server/src/wled_controller/api/routes/system.py` — Add `demo_mode` to system info response
+- `server/src/wled_controller/api/schemas/system.py` — Add `demo_mode` field to response schema
+
+## Acceptance Criteria
+- `Config(demo=True)` is accepted; default is `False`
+- `WLED_DEMO=true` activates demo mode
+- `is_demo_mode()` returns the correct value
+- When demo mode is on, all storage files resolve under `data/demo/`
+- `GET /api/v1/system/info` includes `demo_mode: true/false`
+
+## Notes
+- The env var will be `WLED_DEMO` because of `env_prefix="WLED_"` in pydantic-settings
+- Storage path override should happen at `Config` construction time, not lazily
+
+## Review Checklist
+- [ ] All tasks completed
+- [ ] Code follows project conventions
+- [ ] No unintended side effects
+- [ ] Build passes
+- [ ] Tests pass (new + existing)
+
+## Handoff to Next Phase
+<!-- Filled in after completion -->