Add sync clock entity for synchronized animation timing

Introduces Synchronization Clocks — shared, controllable time bases
that CSS sources can optionally reference for synchronized animation.

Backend:
- New SyncClock dataclass, JSON store, Pydantic schemas, REST API
- Runtime clock with thread-safe pause/resume/reset and speed control
- Ref-counted runtime pool with eager creation for API control
- clock_id field on all ColorStripSource types
- Stream integration: clock time/speed replaces source-local values
- Paused clock skips rendering (saves CPU + stops frame pushes)
- Included in backup/restore via STORE_MAP

Frontend:
- Sync Clocks tab in Streams section with cards and controls
- Clock dropdown in CSS editor (hidden speed slider when clock set)
- Clock crosslink badge on CSS source cards (replaces speed badge)
- Targets tab uses DataCache for picture/audio sources and sync clocks

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

server/src/wled_controller/static/js/app.js

@@ -107,7 +107,7 @@ import {
 // Layer 5: color-strip sources
 import {
     showCSSEditor, closeCSSEditorModal, forceCSSEditorClose, saveCSSEditor, deleteColorStrip,
-    onCSSTypeChange, onEffectTypeChange, onAnimationTypeChange, updateEffectPreview,
+    onCSSTypeChange, onEffectTypeChange, onAnimationTypeChange, onCSSClockChange, updateEffectPreview,
     colorCycleAddColor, colorCycleRemoveColor,
     compositeAddLayer, compositeRemoveLayer,
     mappedAddZone, mappedRemoveZone,
@@ -363,6 +363,7 @@ Object.assign(window, {
     deleteColorStrip,
     onCSSTypeChange,
     onEffectTypeChange,
+    onCSSClockChange,
     onAnimationTypeChange,
     updateEffectPreview,
     colorCycleAddColor,