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/static/locales/en.json

@@ -4,6 +4,8 @@
     "app.api_docs": "API Documentation",
     "app.connection_lost": "Server unreachable",
     "app.connection_retrying": "Attempting to reconnect…",
+    "app.server_restarting": "Server restarting…",
+    "app.server_restarting_sub": "Please wait, the server will be back shortly.",
     "demo.badge": "DEMO",
     "demo.banner": "You're in demo mode — all devices and data are virtual. No real hardware is used.",
     "theme.toggle": "Toggle theme",