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/css/dashboard.css

@@ -218,6 +218,21 @@
     min-width: 48px;
 }
 
+.dashboard-autostart-grid {
+    display: grid;
+    grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
+    gap: 4px;
+}
+
+.dashboard-autostart {
+    grid-template-columns: 1fr auto;
+    margin-bottom: 0;
+}
+
+.dashboard-autostart .dashboard-target-info > div {
+    min-width: 0;
+}
+
 @media (max-width: 768px) {
     .dashboard-target {
         grid-template-columns: 1fr auto;