fix(pwa): persist offline storage + Mapbox offline policy (H8, H9) (#1184)

H8: prefetched tiles and file blobs could be evicted under storage pressure
(worsened by opaque tile responses inflating the quota ~7MB each), blanking the
offline map right when a traveler needs it. Request persistent storage at app
init so the browser exempts our caches from eviction. We deliberately keep tile
requests no-cors (a cors switch would break self-hosted/custom tile providers
without CORS headers), so persistence is the safe mitigation rather than
de-opaquing responses.

H9: Mapbox GL users had no offline map at all — no runtimeCaching matched the
Mapbox hosts. Add a StaleWhileRevalidate rule for api.mapbox.com /
*.tiles.mapbox.com so visited areas are available offline (best-effort; full
pre-download still requires the Leaflet renderer, now documented).

- new sync/persistentStorage.ts requestPersistentStorage(), called from main.tsx
- vite.config: mapbox-tiles SW cache rule
- MapViewAuto / tilePrefetcher comments document the offline-maps policy
- tests for the persist helper (granted / already-persisted / absent / rejects)

client/src/main.tsx

@@ -15,8 +15,11 @@ import '@fontsource/geist-sans/500.css'
 import '@fontsource/geist-sans/600.css'
 import './index.css'
 import { startConnectivityProbe } from './sync/connectivity'
+import { requestPersistentStorage } from './sync/persistentStorage'
 
 startConnectivityProbe()
+// Keep offline data (map tiles, file blobs, IndexedDB) exempt from eviction.
+requestPersistentStorage()
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>