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-6-engine-resolution.md

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+# Phase 6: Demo-only Engine Resolution
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** backend
+
+## Objective
+Ensure demo engines are the primary/preferred engines in demo mode, and are hidden when not in demo mode. This makes demo mode act as a "virtual platform" where only demo engines resolve.
+
+## Tasks
+
+- [ ] Task 1: Modify `EngineRegistry.get_available_engines()` to filter out engines with `ENGINE_TYPE == "demo"` when not in demo mode (they report `is_available()=False` anyway, but belt-and-suspenders)
+- [ ] Task 2: Modify `AudioEngineRegistry.get_available_engines()` similarly
+- [ ] Task 3: In demo mode, `get_best_available_engine()` should return the demo engine (already handled by priority=1000, but verify)
+- [ ] Task 4: Modify the `GET /api/v1/config/displays` endpoint: in demo mode, default to demo engine displays if no engine_type specified
+- [ ] Task 5: Modify the audio engine listing endpoint similarly
+- [ ] Task 6: Ensure `DemoDeviceProvider.discover()` only returns devices when in demo mode
+- [ ] Task 7: End-to-end verification: start server in demo mode, verify only demo engines/devices appear in API responses
+
+## Files to Modify/Create
+- `server/src/wled_controller/core/capture_engines/factory.py` — Filter demo engines
+- `server/src/wled_controller/core/audio/factory.py` — Filter demo engines
+- `server/src/wled_controller/api/routes/system.py` — Display endpoint defaults
+- `server/src/wled_controller/api/routes/audio_templates.py` — Audio engine listing
+- `server/src/wled_controller/core/devices/demo_provider.py` — Guard discover()
+
+## Acceptance Criteria
+- In demo mode: demo engines are primary, real engines may also be listed but demo is default
+- Not in demo mode: demo engines are completely hidden from all API responses
+- Display list defaults to demo displays in demo mode
+- Audio device list defaults to demo devices in demo mode
+
+## Notes
+- This is the "demo OS identifier" concept — demo mode acts as a virtual platform
+- Be careful not to break existing behavior when demo=False (default)
+- The demo engines already have `is_available() = is_demo_mode()`, so the main concern is UI defaults
+
+## 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 -->