Add WLED health monitoring, calibration test mode, and UI improvements

- Add background health checks (GET /json/info) with configurable interval per device
- Auto-detect LED count from WLED device on add (remove led_count from create API)
- Add calibration test mode: toggle edges on/off with colored LEDs via PUT endpoint
- Show WLED firmware version badge and LED count badge on device cards
- Add modal dirty tracking with discard confirmation on close/backdrop click
- Fix layout jump when modals open by compensating for scrollbar width
- Add state_check_interval to settings API and UI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/main.py

@@ -78,6 +78,9 @@ async def lifespan(app: FastAPI):
 
     logger.info(f"Loaded {len(devices)} devices from storage")
 
+    # Start background health monitoring for all devices
+    await processor_manager.start_health_monitoring()
+
     yield
 
     # Shutdown