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>

server/config/demo_config.yaml

@@ -0,0 +1,34 @@
+# Demo mode configuration
+# Loaded automatically when WLED_DEMO=true is set.
+# Uses isolated data directory (data/demo/) and a pre-configured API key
+# so the demo works out of the box with zero setup.
+
+demo: true
+
+server:
+  host: "0.0.0.0"
+  port: 8081
+  log_level: "INFO"
+  cors_origins:
+    - "*"
+
+auth:
+  api_keys:
+    demo: "demo"
+
+storage:
+  devices_file: "data/devices.json"
+  templates_file: "data/capture_templates.json"
+  postprocessing_templates_file: "data/postprocessing_templates.json"
+  picture_sources_file: "data/picture_sources.json"
+  output_targets_file: "data/output_targets.json"
+  pattern_templates_file: "data/pattern_templates.json"
+
+mqtt:
+  enabled: false
+
+logging:
+  format: "text"
+  file: "logs/wled_controller.log"
+  max_size_mb: 100
+  backup_count: 5