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/perf-charts.js

@@ -180,3 +180,14 @@ export function stopPerfPolling() {
         _pollTimer = null;
     }
 }
+
+// Pause polling when browser tab becomes hidden, resume when visible
+document.addEventListener('visibilitychange', () => {
+    if (document.hidden) {
+        stopPerfPolling();
+    } else {
+        // Only resume if dashboard is active
+        const activeTab = localStorage.getItem('activeTab') || 'dashboard';
+        if (activeTab === 'dashboard') startPerfPolling();
+    }
+});