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/__init__.py

@@ -17,6 +17,7 @@ from .routes.value_sources import router as value_sources_router
 from .routes.automations import router as automations_router
 from .routes.scene_presets import router as scene_presets_router
 from .routes.webhooks import router as webhooks_router
+from .routes.sync_clocks import router as sync_clocks_router
 
 router = APIRouter()
 router.include_router(system_router)
@@ -34,5 +35,6 @@ router.include_router(picture_targets_router)
 router.include_router(automations_router)
 router.include_router(scene_presets_router)
 router.include_router(webhooks_router)
+router.include_router(sync_clocks_router)
 
 __all__ = ["router"]