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/static/js/features/kc-targets.js

@@ -219,7 +219,7 @@ export function toggleKCTestAutoRefresh() {
             } catch (e) {
                 stopKCTestAutoRefresh();
             }
-        }, 1000));
+        }, 2000));
         updateAutoRefreshButton(true);
     }
 }