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/value-sources.js

@@ -10,7 +10,7 @@
  * This module manages the editor modal and API operations.
  */
 
-import { _cachedValueSources, set_cachedValueSources, _cachedAudioSources, _cachedStreams, apiKey } from '../core/state.js';
+import { _cachedValueSources, _cachedAudioSources, _cachedStreams, apiKey } from '../core/state.js';
 import { API_BASE, fetchWithAuth, escapeHtml } from '../core/api.js';
 import { t } from '../core/i18n.js';
 import { showToast, showConfirm } from '../core/ui.js';