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/api/routes/system.py

@@ -271,6 +271,7 @@ STORE_MAP = {
     "audio_sources": "audio_sources_file",
     "audio_templates": "audio_templates_file",
     "value_sources": "value_sources_file",
+    "sync_clocks": "sync_clocks_file",
     "automations": "automations_file",
     "scene_presets": "scene_presets_file",
 }