feat: graceful shutdown with store persistence and restart overlay

- Add /api/v1/system/shutdown endpoint that triggers clean uvicorn exit
- Persist all 15 stores to disk during shutdown via _save_all_stores()
- Add force parameter to BaseJsonStore._save() to bypass restore freeze
- Restart script now requests graceful shutdown via API (15s timeout),
  falls back to force-kill only if server doesn't exit in time
- Broadcast server_restarting event over WebSocket before shutdown
- Frontend shows "Server restarting..." overlay instantly on WS event,
  replacing the old dynamically-created overlay from settings.ts
- Add server_ref module to share uvicorn Server + TrayManager refs
- Add i18n keys for restart overlay (en/ru/zh)

server/src/wled_controller/api/routes/backup.py

@@ -228,10 +228,25 @@ def backup_config(_: AuthRequired):
 @router.post("/api/v1/system/restart", tags=["System"])
 def restart_server(_: AuthRequired):
     """Schedule a server restart and return immediately."""
+    from wled_controller.server_ref import _broadcast_restarting
+    _broadcast_restarting()
     _schedule_restart()
     return {"status": "restarting"}
 
 
+@router.post("/api/v1/system/shutdown", tags=["System"])
+def shutdown_server(_: AuthRequired):
+    """Gracefully shut down the server.
+
+    Signals uvicorn to exit, which triggers the lifespan shutdown handler
+    (persists all stores to disk, stops processors, etc.).
+    Used by the restart script to ensure data is saved before the process exits.
+    """
+    from wled_controller.server_ref import request_shutdown
+    request_shutdown()
+    return {"status": "shutting_down"}
+
+
 @router.post("/api/v1/system/restore", response_model=RestoreResponse, tags=["System"])
 async def restore_config(
     _: AuthRequired,