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/features/targets.js

@@ -1003,6 +1003,10 @@ function disconnectLedPreviewWS(targetId) {
     if (panel) panel.style.display = 'none';
 }
 
+export function disconnectAllLedPreviewWS() {
+    Object.keys(ledPreviewWebSockets).forEach(id => disconnectLedPreviewWS(id));
+}
+
 export function toggleLedPreview(targetId) {
     const panel = document.getElementById(`led-preview-panel-${targetId}`);
     if (!panel) return;