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/streams.js

@@ -20,6 +20,7 @@ import {
     _lastValidatedImageSource, set_lastValidatedImageSource,
     _cachedAudioSources, set_cachedAudioSources,
     _cachedValueSources, set_cachedValueSources,
+    _sourcesLoading, set_sourcesLoading,
     apiKey,
 } from '../core/state.js';
 import { API_BASE, getHeaders, fetchWithAuth, escapeHtml } from '../core/api.js';
@@ -129,7 +130,9 @@ async function loadCaptureTemplates() {
         set_cachedCaptureTemplates(data.templates || []);
         renderPictureSourcesList(_cachedStreams);
     } catch (error) {
+        if (error.isAuth) return;
         console.error('Error loading capture templates:', error);
+        showToast(t('streams.error.load'), 'error');
     }
 }
 
@@ -511,6 +514,8 @@ export async function deleteTemplate(templateId) {
 // ===== Picture Sources =====
 
 export async function loadPictureSources() {
+    if (_sourcesLoading) return;
+    set_sourcesLoading(true);
     try {
         const [filtersResp, ppResp, captResp, streamsResp, audioResp, valueResp] = await Promise.all([
             _availableFilters.length === 0 ? fetchWithAuth('/filters') : Promise.resolve(null),
@@ -546,10 +551,13 @@ export async function loadPictureSources() {
         set_cachedStreams(data.streams || []);
         renderPictureSourcesList(_cachedStreams);
     } catch (error) {
+        if (error.isAuth) return;
         console.error('Error loading picture sources:', error);
         document.getElementById('streams-list').innerHTML = `
             <div class="error-message">${t('streams.error.load')}: ${error.message}</div>
         `;
+    } finally {
+        set_sourcesLoading(false);
     }
 }