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

@@ -92,6 +92,29 @@ processor_manager = ProcessorManager(
 )
 
 
+def _save_all_stores() -> None:
+    """Persist every store to disk.
+
+    Called during graceful shutdown to ensure in-memory data survives
+    restarts even if no CRUD happened during the session.
+    """
+    all_stores = [
+        device_store, template_store, pp_template_store,
+        picture_source_store, output_target_store, pattern_template_store,
+        color_strip_store, audio_source_store, audio_template_store,
+        value_source_store, automation_store, scene_preset_store,
+        sync_clock_store, cspt_store, gradient_store,
+    ]
+    saved = 0
+    for store in all_stores:
+        try:
+            store._save(force=True)
+            saved += 1
+        except Exception as e:
+            logger.error(f"Failed to save {store._json_key} on shutdown: {e}")
+    logger.info(f"Shutdown save: persisted {saved}/{len(all_stores)} stores to disk")
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Application lifespan manager.
@@ -230,6 +253,11 @@ async def lifespan(app: FastAPI):
     # Shutdown
     logger.info("Shutting down LED Grab")
 
+    # Persist all stores to disk before stopping anything.
+    # This ensures in-memory data survives force-kills and restarts
+    # where no CRUD happened during the session.
+    _save_all_stores()
+
     # Stop auto-backup engine
     try:
         await auto_backup_engine.stop()