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/storage/sync_clock.py

@@ -0,0 +1,43 @@
+"""Synchronization clock data model.
+
+A SyncClock provides a shared, controllable time base for animation-based
+color strip sources.  Multiple CSS sources referencing the same clock
+animate in sync and share speed / pause / resume / reset controls.
+"""
+
+from dataclasses import dataclass
+from datetime import datetime
+from typing import Optional
+
+
+@dataclass
+class SyncClock:
+    """Persistent configuration for a synchronization clock."""
+
+    id: str
+    name: str
+    speed: float  # animation speed multiplier (0.1–10.0)
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "name": self.name,
+            "speed": self.speed,
+            "description": self.description,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }
+
+    @staticmethod
+    def from_dict(data: dict) -> "SyncClock":
+        return SyncClock(
+            id=data["id"],
+            name=data["name"],
+            speed=float(data.get("speed", 1.0)),
+            description=data.get("description"),
+            created_at=datetime.fromisoformat(data["created_at"]),
+            updated_at=datetime.fromisoformat(data["updated_at"]),
+        )