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/server_ref.py

@@ -0,0 +1,59 @@
+"""Module-level holder for the uvicorn Server and TrayManager references.
+
+Allows the shutdown API endpoint to trigger graceful shutdown via
+``server.should_exit = True`` + ``tray.stop()``, which is the same
+mechanism the system tray "Shutdown" menu item uses.
+"""
+
+from typing import Any, Optional
+
+_server: Optional[Any] = None  # uvicorn.Server
+_tray: Optional[Any] = None    # TrayManager
+
+
+def set_server(server: Any) -> None:
+    """Store the uvicorn Server instance (called from __main__)."""
+    global _server
+    _server = server
+
+
+def set_tray(tray: Any) -> None:
+    """Store the TrayManager instance (called from __main__)."""
+    global _tray
+    _tray = tray
+
+
+def request_shutdown() -> None:
+    """Signal uvicorn + tray to perform a graceful shutdown.
+
+    Broadcasts a ``server_restarting`` event so the frontend can show
+    a restart indicator, persists all stores to disk, then sets
+    ``should_exit = True`` on the uvicorn Server and stops the tray.
+    """
+    # Notify connected clients that a restart is in progress
+    _broadcast_restarting()
+
+    # Persist stores before signaling shutdown.
+    # The lifespan shutdown handler also saves, but it may not run
+    # reliably when uvicorn is in a daemon thread.
+    try:
+        from wled_controller.main import _save_all_stores
+        _save_all_stores()
+    except Exception:
+        pass  # best-effort; lifespan handler is the backup
+
+    if _server is not None:
+        _server.should_exit = True
+    if _tray is not None:
+        _tray.stop()
+
+
+def _broadcast_restarting() -> None:
+    """Push a server_restarting event to all connected WebSocket clients."""
+    try:
+        from wled_controller.api.dependencies import _deps
+        pm = _deps.get("processor_manager")
+        if pm is not None:
+            pm.fire_event({"type": "server_restarting"})
+    except Exception:
+        pass