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/kc-targets.js

@@ -8,7 +8,7 @@ import {
     _kcNameManuallyEdited, set_kcNameManuallyEdited,
     kcWebSockets,
     PATTERN_RECT_BORDERS,
-    _cachedValueSources, set_cachedValueSources,
+    _cachedValueSources, valueSourcesCache,
 } from '../core/state.js';
 import { API_BASE, getHeaders, fetchWithAuth, escapeHtml } from '../core/api.js';
 import { t } from '../core/i18n.js';
@@ -413,15 +413,13 @@ function _populateKCBrightnessVsDropdown(selectedId = '') {
 export async function showKCEditor(targetId = null, cloneData = null) {
     try {
         // Load sources, pattern templates, and value sources in parallel
-        const [sourcesResp, patResp, vsResp] = await Promise.all([
+        const [sourcesResp, patResp, valueSources] = await Promise.all([
             fetchWithAuth('/picture-sources').catch(() => null),
             fetchWithAuth('/pattern-templates').catch(() => null),
-            fetchWithAuth('/value-sources').catch(() => null),
+            valueSourcesCache.fetch(),
         ]);
         const sources = (sourcesResp && sourcesResp.ok) ? (await sourcesResp.json()).streams || [] : [];
         const patTemplates = (patResp && patResp.ok) ? (await patResp.json()).templates || [] : [];
-        const valueSources = (vsResp && vsResp.ok) ? (await vsResp.json()).sources || [] : [];
-        set_cachedValueSources(valueSources);
 
         // Populate source select
         const sourceSelect = document.getElementById('kc-editor-source');