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-2-virtual-capture-engine.md

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+# Phase 2: Virtual Capture Engine
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** backend
+
+## Objective
+Create a `DemoCaptureEngine` that provides virtual displays and produces animated test pattern frames, allowing screen capture workflows to function in demo mode without real monitors.
+
+## Tasks
+
+- [ ] Task 1: Create `server/src/wled_controller/core/capture_engines/demo_engine.py` with `DemoCaptureEngine` and `DemoCaptureStream`
+- [ ] Task 2: `DemoCaptureEngine.ENGINE_TYPE = "demo"`, `ENGINE_PRIORITY = 1000` (highest in demo mode)
+- [ ] Task 3: `is_available()` returns `True` only when `is_demo_mode()` is True
+- [ ] Task 4: `get_available_displays()` returns 3 virtual displays:
+  - "Demo Display 1080p" (1920×1080)
+  - "Demo Ultrawide" (3440×1440)
+  - "Demo Portrait" (1080×1920)
+- [ ] Task 5: `DemoCaptureStream.capture_frame()` produces animated test patterns:
+  - Horizontally scrolling rainbow gradient (simple, visually clear)
+  - Uses `time.time()` for animation so frames change over time
+  - Returns proper `ScreenCapture` with RGB numpy array
+- [ ] Task 6: Register `DemoCaptureEngine` in `capture_engines/__init__.py`
+
+## Files to Modify/Create
+- `server/src/wled_controller/core/capture_engines/demo_engine.py` — New file: DemoCaptureEngine + DemoCaptureStream
+- `server/src/wled_controller/core/capture_engines/__init__.py` — Register DemoCaptureEngine
+
+## Acceptance Criteria
+- `DemoCaptureEngine.is_available()` is True only in demo mode
+- Virtual displays appear in the display list API when in demo mode
+- `capture_frame()` returns valid RGB frames that change over time
+- Engine is properly registered in EngineRegistry
+
+## Notes
+- Test patterns should be computationally cheap (no heavy image processing)
+- Use numpy operations for pattern generation (vectorized, fast)
+- Frame dimensions must match the virtual display dimensions
+
+## 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 -->