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/locales/en.json

@@ -307,6 +307,7 @@
   "common.delete": "Delete",
   "common.edit": "Edit",
   "common.clone": "Clone",
+  "common.none": "None",
   "section.filter.placeholder": "Filter...",
   "section.filter.reset": "Clear filter",
   "section.expand_all": "Expand all sections",
@@ -950,6 +951,7 @@
   "audio_template.error.required": "Please fill in all required fields",
   "audio_template.error.delete": "Failed to delete audio template",
   "streams.group.value": "Value Sources",
+  "streams.group.sync": "Sync Clocks",
   "value_source.group.title": "Value Sources",
   "value_source.add": "Add Value Source",
   "value_source.edit": "Edit Value Source",
@@ -1137,5 +1139,32 @@
   "theme.switched.dark": "Switched to dark theme",
   "theme.switched.light": "Switched to light theme",
   "accent.color.updated": "Accent color updated",
-  "search.footer": "↑↓ navigate · Enter select · Esc close"
+  "search.footer": "↑↓ navigate · Enter select · Esc close",
+  "sync_clock.group.title": "Sync Clocks",
+  "sync_clock.add": "Add Sync Clock",
+  "sync_clock.edit": "Edit Sync Clock",
+  "sync_clock.name": "Name:",
+  "sync_clock.name.placeholder": "Main Animation Clock",
+  "sync_clock.name.hint": "A descriptive name for this synchronization clock",
+  "sync_clock.speed": "Speed:",
+  "sync_clock.speed.hint": "Speed multiplier shared by all linked sources. 1.0 = normal speed.",
+  "sync_clock.description": "Description (optional):",
+  "sync_clock.description.placeholder": "Optional description",
+  "sync_clock.description.hint": "Optional notes about this clock's purpose",
+  "sync_clock.status.running": "Running",
+  "sync_clock.status.paused": "Paused",
+  "sync_clock.action.pause": "Pause",
+  "sync_clock.action.resume": "Resume",
+  "sync_clock.action.reset": "Reset",
+  "sync_clock.error.name_required": "Clock name is required",
+  "sync_clock.error.load": "Failed to load sync clock",
+  "sync_clock.created": "Sync clock created",
+  "sync_clock.updated": "Sync clock updated",
+  "sync_clock.deleted": "Sync clock deleted",
+  "sync_clock.paused": "Clock paused",
+  "sync_clock.resumed": "Clock resumed",
+  "sync_clock.reset_done": "Clock reset to zero",
+  "sync_clock.delete.confirm": "Delete this sync clock? Sources using it will revert to their own speed.",
+  "color_strip.clock": "Sync Clock:",
+  "color_strip.clock.hint": "Link to a sync clock for synchronized animation. When set, speed comes from the clock."
 }