Add reusable DataCache class, unify frontend cache patterns

- Create DataCache class with fetch deduplication, invalidation, subscribers
- Instantiate 10 cache instances in state.js (streams, templates, sources, etc.)
- Replace inline fetch+parse+set patterns with cache.fetch() calls across modules
- Eliminate dual _scenesCache/_presetsCache sync via shared scenePresetsCache
- Remove 9 now-unused setter functions from state.js
- Clean up unused setter imports from audio-sources, value-sources, displays

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/features/displays.js

@@ -5,10 +5,9 @@
 
 import {
     _cachedDisplays, _displayPickerCallback, _displayPickerSelectedIndex,
-    set_displayPickerCallback, set_displayPickerSelectedIndex, set_cachedDisplays,
+    set_displayPickerCallback, set_displayPickerSelectedIndex, displaysCache,
 } from '../core/state.js';
 import { t } from '../core/i18n.js';
-import { loadDisplays } from '../core/api.js';
 import { fetchWithAuth } from '../core/api.js';
 import { showToast } from '../core/ui.js';
 
@@ -32,14 +31,12 @@ export function openDisplayPicker(callback, selectedIndex, engineType = null) {
         } else {
             const canvas = document.getElementById('display-picker-canvas');
             canvas.innerHTML = '<div class="loading-spinner"></div>';
-            loadDisplays().then(() => {
-                import('../core/state.js').then(({ _cachedDisplays: displays }) => {
-                    if (displays && displays.length > 0) {
-                        renderDisplayPickerLayout(displays);
-                    } else {
-                        canvas.innerHTML = `<div class="loading">${t('displays.none')}</div>`;
-                    }
-                });
+            displaysCache.fetch().then(displays => {
+                if (displays && displays.length > 0) {
+                    renderDisplayPickerLayout(displays);
+                } else {
+                    canvas.innerHTML = `<div class="loading">${t('displays.none')}</div>`;
+                }
             });
         }
     });
@@ -56,7 +53,7 @@ async function _fetchAndRenderEngineDisplays(engineType) {
         const displays = data.displays || [];
 
         // Store in cache so selectDisplay() can look them up
-        set_cachedDisplays(displays);
+        displaysCache.update(displays);
 
         if (displays.length > 0) {
             renderDisplayPickerLayout(displays, engineType);