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/kc-targets.js

@@ -607,6 +607,7 @@ export async function saveKCBrightness(targetId, value) {
         });
     } catch (err) {
         console.error('Failed to save KC brightness:', err);
+        showToast(t('kc.error.brightness') || 'Failed to save brightness', 'error');
     }
 }