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/pattern-templates.js

@@ -36,6 +36,13 @@ class PatternTemplateModal extends Modal {
         setPatternEditorRects([]);
         setPatternEditorSelectedIdx(-1);
         setPatternEditorBgImage(null);
+        // Clean up ResizeObserver to prevent leaks
+        const canvas = document.getElementById('pattern-canvas');
+        if (canvas?._patternResizeObserver) {
+            canvas._patternResizeObserver.disconnect();
+            canvas._patternResizeObserver = null;
+        }
+        if (canvas) canvas._patternEventsAttached = false;
     }
 }