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/__main__.py

@@ -15,6 +15,7 @@ from pathlib import Path
 import uvicorn
 
 from wled_controller.config import get_config
+from wled_controller.server_ref import set_server, set_tray
 from wled_controller.tray import PYSTRAY_AVAILABLE, TrayManager
 from wled_controller.utils import setup_logging, get_logger
 
@@ -52,6 +53,7 @@ def main() -> None:
         log_level=config.server.log_level.lower(),
     )
     server = uvicorn.Server(uv_config)
+    set_server(server)
 
     use_tray = PYSTRAY_AVAILABLE and (
         sys.platform == "win32" or _force_tray()
@@ -80,6 +82,7 @@ def main() -> None:
             port=config.server.port,
             on_exit=lambda: _request_shutdown(server),
         )
+        set_tray(tray)
         tray.run()
 
         # Tray exited — wait for server to finish its graceful shutdown