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/core/processing/processor_manager.py

@@ -407,6 +407,18 @@ class ProcessorManager:
         """Get detailed metrics for a target (any type)."""
         return self._get_processor(target_id).get_metrics()
 
+    def get_all_target_states(self) -> Dict[str, dict]:
+        """Get processing state for all targets (with device health merged)."""
+        return {tid: self.get_target_state(tid) for tid in self._processors}
+
+    def get_all_target_metrics(self) -> Dict[str, dict]:
+        """Get metrics for all targets."""
+        return {tid: proc.get_metrics() for tid, proc in self._processors.items()}
+
+    def get_all_device_health_dicts(self) -> Dict[str, dict]:
+        """Get health/connection state for all devices."""
+        return {did: self.get_device_health_dict(did) for did in self._devices}
+
     def is_target_processing(self, target_id: str) -> bool:
         """Check if target is currently processing."""
         return self._get_processor(target_id).is_running