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/PLAN.md

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+# Feature: Demo Mode
+
+**Branch:** `feature/demo-mode`
+**Base branch:** `master`
+**Created:** 2026-03-20
+**Status:** 🟡 In Progress
+**Strategy:** Big Bang
+**Mode:** Automated
+**Execution:** Orchestrator
+
+## Summary
+Add a demo mode that allows users to explore and test the app without real hardware. Virtual capture engines, audio engines, and device providers replace real hardware. An isolated data directory with seed data provides a fully populated sandbox. A visual indicator in the UI makes it clear the app is running in demo mode.
+
+## Build & Test Commands
+- **Build (frontend):** `cd server && npm run build`
+- **Typecheck (frontend):** `cd server && npm run typecheck`
+- **Test (backend):** `cd server && python -m pytest ../tests/ -x`
+- **Server start:** `cd server && python -m wled_controller.main`
+
+## Phases
+
+- [x] Phase 1: Demo Mode Config & Flag [domain: backend] → [subplan](./phase-1-config-flag.md)
+- [x] Phase 2: Virtual Capture Engine [domain: backend] → [subplan](./phase-2-virtual-capture-engine.md)
+- [x] Phase 3: Virtual Audio Engine [domain: backend] → [subplan](./phase-3-virtual-audio-engine.md)
+- [x] Phase 4: Demo Device Provider & Seed Data [domain: backend] → [subplan](./phase-4-demo-device-seed-data.md)
+- [x] Phase 5: Frontend Demo Indicator & Sandbox UX [domain: fullstack] → [subplan](./phase-5-frontend-demo-ux.md)
+- [x] Phase 6: Demo-only Engine Resolution [domain: backend] → [subplan](./phase-6-engine-resolution.md)
+
+## Phase Progress Log
+
+| Phase | Domain | Status | Review | Build | Committed |
+|-------|--------|--------|--------|-------|-----------|
+| Phase 1: Config & Flag | backend | ✅ Done | ✅ | ✅ | ⬜ |
+| Phase 2: Virtual Capture Engine | backend | ✅ Done | ✅ | ✅ | ⬜ |
+| Phase 3: Virtual Audio Engine | backend | ✅ Done | ✅ | ✅ | ⬜ |
+| Phase 4: Demo Device & Seed Data | backend | ✅ Done | ✅ | ✅ | ⬜ |
+| Phase 5: Frontend Demo UX | fullstack | ✅ Done | ✅ | ✅ | ⬜ |
+| Phase 6: Engine Resolution | backend | ✅ Done | ✅ | ✅ | ⬜ |
+
+## Final Review
+- [ ] Comprehensive code review
+- [ ] Full build passes
+- [ ] Full test suite passes
+- [ ] Merged to `master`