Batch API endpoints, reduce frontend polling by ~75%, fix resource leaks

Backend: add batch endpoints for target states, metrics, and device
health to replace O(N) individual API calls per poll cycle.
Frontend: use batch endpoints in dashboard/targets/profiles tabs,
fix Chart.js instance leaks, debounce server event reloads, add
i18n active-tab guards, clean up ResizeObserver on pattern editor
close, cache uptime timer DOM refs, increase KC auto-refresh to 2s.

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

server/src/wled_controller/api/routes/devices.py

@@ -202,6 +202,15 @@ async def discover_devices(
     )
 
 
+@router.get("/api/v1/devices/batch/states", tags=["Devices"])
+async def batch_device_states(
+    _auth: AuthRequired,
+    manager: ProcessorManager = Depends(get_processor_manager),
+):
+    """Get health/connection state for all devices in a single request."""
+    return {"states": manager.get_all_device_health_dicts()}
+
+
 @router.get("/api/v1/devices/{device_id}", response_model=DeviceResponse, tags=["Devices"])
 async def get_device(
     device_id: str,