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/scene-presets.js

@@ -11,14 +11,11 @@ import { CardSection } from '../core/card-sections.js';
 import {
     ICON_SCENE, ICON_CAPTURE, ICON_START, ICON_EDIT, ICON_REFRESH, ICON_TARGET,
 } from '../core/icons.js';
+import { scenePresetsCache } from '../core/state.js';
 
-let _presetsCache = [];
 let _editingId = null;
 let _allTargets = [];  // fetched on capture open
 
-/** Update the internal presets cache (called from automations tab after fetching). */
-export function updatePresetsCache(presets) { _presetsCache = presets; }
-
 class ScenePresetEditorModal extends Modal {
     constructor() { super('scene-preset-editor-modal'); }
     snapshotValues() {
@@ -74,15 +71,7 @@ export function createSceneCard(preset) {
 // ===== Dashboard section (compact cards) =====
 
 export async function loadScenePresets() {
-    try {
-        const resp = await fetchWithAuth('/scene-presets');
-        if (!resp.ok) return [];
-        const data = await resp.json();
-        _presetsCache = data.presets || [];
-        return _presetsCache;
-    } catch {
-        return [];
-    }
+    return scenePresetsCache.fetch();
 }
 
 export function renderScenePresetsSection(presets) {
@@ -153,7 +142,7 @@ export async function openScenePresetCapture() {
 // ===== Edit metadata =====
 
 export async function editScenePreset(presetId) {
-    const preset = _presetsCache.find(p => p.id === presetId);
+    const preset = scenePresetsCache.data.find(p => p.id === presetId);
     if (!preset) return;
 
     _editingId = presetId;
@@ -295,7 +284,7 @@ export async function activateScenePreset(presetId) {
 // ===== Recapture =====
 
 export async function recaptureScenePreset(presetId) {
-    const preset = _presetsCache.find(p => p.id === presetId);
+    const preset = scenePresetsCache.data.find(p => p.id === presetId);
     const name = preset ? preset.name : presetId;
     const confirmed = await showConfirm(t('scenes.recapture_confirm', { name }));
     if (!confirmed) return;
@@ -319,7 +308,7 @@ export async function recaptureScenePreset(presetId) {
 // ===== Delete =====
 
 export async function deleteScenePreset(presetId) {
-    const preset = _presetsCache.find(p => p.id === presetId);
+    const preset = scenePresetsCache.data.find(p => p.id === presetId);
     const name = preset ? preset.name : presetId;
     const confirmed = await showConfirm(t('scenes.delete_confirm', { name }));
     if (!confirmed) return;