Remove PWA/SW and fix blocking startup I/O

- Remove vite-plugin-pwa and all service worker code. The app no longer
  registers a service worker; legacy SWs are unregistered on load.
- Move snapshot parsing (thousands of sync Path.exists() calls) into a
  thread pool via asyncio.to_thread() so the health check responds
  immediately on startup.
- Wrap aiopathlib AsyncPath.iterdir() (sync os.scandir under the hood)
  in asyncio.to_thread() in scanner and scanning modules so directory
  walks do not monopolize the event loop.

frontend/src/main.ts

@@ -11,10 +11,13 @@ installInputModalityTracking()
 installKeyboardNavigation()
 installGamepadNavigation()
 
-if ('serviceWorker' in navigator && !navigator.serviceWorker.controller) {
-	navigator.serviceWorker.addEventListener('controllerchange', () => {
-		window.location.reload()
-	}, { once: true })
+// Unregister any legacy service workers — MediaHive no longer uses a PWA/SW.
+if ('serviceWorker' in navigator) {
+	navigator.serviceWorker.getRegistrations().then((registrations) => {
+		for (const registration of registrations) {
+			registration.unregister()
+		}
+	})
 }
 
 createApp(App).use(router).mount('#app')