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-3-virtual-audio-engine.md

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+# Phase 3: Virtual Audio Engine
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** backend
+
+## Objective
+Create a `DemoAudioEngine` that provides virtual audio devices and produces synthetic audio data, enabling audio-reactive visualizations in demo mode.
+
+## Tasks
+
+- [ ] Task 1: Create `server/src/wled_controller/core/audio/demo_engine.py` with `DemoAudioEngine` and `DemoAudioCaptureStream`
+- [ ] Task 2: `DemoAudioEngine.ENGINE_TYPE = "demo"`, `ENGINE_PRIORITY = 1000`
+- [ ] Task 3: `is_available()` returns `True` only when `is_demo_mode()` is True
+- [ ] Task 4: `enumerate_devices()` returns 2 virtual devices:
+  - "Demo Microphone" (input, not loopback)
+  - "Demo System Audio" (loopback)
+- [ ] Task 5: `DemoAudioCaptureStream` implements:
+  - `channels = 2`, `sample_rate = 44100`, `chunk_size = 1024`
+  - `read_chunk()` produces synthetic audio: a mix of sine waves with slowly varying frequencies to simulate music-like beat patterns
+  - Returns proper float32 ndarray
+- [ ] Task 6: Register `DemoAudioEngine` in `audio/__init__.py`
+
+## Files to Modify/Create
+- `server/src/wled_controller/core/audio/demo_engine.py` — New file: DemoAudioEngine + DemoAudioCaptureStream
+- `server/src/wled_controller/core/audio/__init__.py` — Register DemoAudioEngine
+
+## Acceptance Criteria
+- `DemoAudioEngine.is_available()` is True only in demo mode
+- Virtual audio devices appear in audio device enumeration when in demo mode
+- `read_chunk()` returns valid float32 audio data that varies over time
+- Audio analyzer produces non-trivial frequency band data from the synthetic signal
+
+## Notes
+- Synthetic audio should produce interesting FFT results (multiple frequencies, amplitude modulation)
+- Keep it computationally lightweight
+- Must conform to `AudioCaptureStreamBase` interface exactly
+
+## 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 -->