Fix critical frontend issues: race conditions, memory leaks, silent failures

- Add loading guard to loadPictureSources to prevent concurrent fetches
- Pause perf chart polling and uptime timer when browser tab is hidden
- Disconnect KC and LED preview WebSockets when leaving targets tab
- Add error toasts to loadCaptureTemplates and saveKCBrightness
- Skip auto-refresh polling when document is hidden
- Widen auto-start dashboard cards for better text display

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

server/src/wled_controller/static/js/app.js

@@ -90,6 +90,7 @@ import {
     startTargetOverlay, stopTargetOverlay, deleteTarget,
     cloneTarget, toggleLedPreview, toggleTargetAutoStart,
     expandAllTargetSections, collapseAllTargetSections,
+    disconnectAllLedPreviewWS,
 } from './features/targets.js';
 
 // Layer 5: color-strip sources
@@ -303,6 +304,7 @@ Object.assign(window, {
     cloneTarget,
     toggleLedPreview,
     toggleTargetAutoStart,
+    disconnectAllLedPreviewWS,
 
     // color-strip sources
     showCSSEditor,
@@ -413,6 +415,7 @@ window.addEventListener('beforeunload', () => {
     }
     stopEventsWS();
     disconnectAllKCWebSockets();
+    disconnectAllLedPreviewWS();
 });
 
 // ─── Initialization ───