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-4-demo-device-seed-data.md

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+# Phase 4: Demo Device Provider & Seed Data
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** backend
+
+## Objective
+Create a demo device provider that exposes discoverable virtual LED devices, and build a seed data generator that populates the demo data directory with sample entities on first run.
+
+## Tasks
+
+- [ ] Task 1: Create `server/src/wled_controller/core/devices/demo_provider.py` — `DemoDeviceProvider` extending `LEDDeviceProvider`:
+  - `device_type = "demo"`
+  - `capabilities = {"manual_led_count", "power_control", "brightness_control", "static_color"}`
+  - `create_client()` returns a `MockClient` (reuse existing)
+  - `discover()` returns 3 pre-defined virtual devices:
+    - "Demo LED Strip" (60 LEDs, ip="demo-strip")
+    - "Demo LED Matrix" (256 LEDs / 16×16, ip="demo-matrix")
+    - "Demo LED Ring" (24 LEDs, ip="demo-ring")
+  - `check_health()` always returns online with simulated ~2ms latency
+  - `validate_device()` returns `{"led_count": <from url>}`
+- [ ] Task 2: Register `DemoDeviceProvider` in `led_client.py` `_register_builtin_providers()`
+- [ ] Task 3: Create `server/src/wled_controller/core/demo_seed.py` — seed data generator:
+  - Function `seed_demo_data(storage_config: StorageConfig)` that checks if demo data dir is empty and populates it
+  - Seed entities: 3 devices (matching discover results), 2 output targets, 2 picture sources (using demo engine), 2 CSS sources (gradient + color_cycle), 1 audio source (using demo engine), 1 scene preset, 1 automation
+  - Use proper ID formats matching existing conventions (e.g., `dev_<hex>`, `tgt_<hex>`, etc.)
+- [ ] Task 4: Call `seed_demo_data()` during server startup in `main.py` when demo mode is active (before stores are loaded)
+
+## Files to Modify/Create
+- `server/src/wled_controller/core/devices/demo_provider.py` — New: DemoDeviceProvider
+- `server/src/wled_controller/core/devices/led_client.py` — Register DemoDeviceProvider
+- `server/src/wled_controller/core/demo_seed.py` — New: seed data generator
+- `server/src/wled_controller/main.py` — Call seed on demo startup
+
+## Acceptance Criteria
+- Demo devices appear in discovery results when in demo mode
+- Seed data populates `data/demo/` with valid JSON files on first demo run
+- Subsequent demo runs don't overwrite existing demo data
+- All seeded entities load correctly in stores
+
+## Notes
+- Seed data must match the exact schema expected by each store (look at existing JSON files for format)
+- Use the entity dataclass `to_dict()` / store patterns to generate valid data
+- Demo discovery should NOT appear when not in demo mode
+
+## 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 -->