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/components/Map/MapViewAuto.tsx

@@ -5,6 +5,11 @@ import { MapViewGL } from './MapViewGL'
 // Auto-selects the map renderer based on user settings. Keeps the existing
 // Leaflet MapView untouched so the Mapbox GL variant can mature iteratively
 // behind a toggle. Atlas is not affected — it imports Leaflet directly.
+//
+// Offline maps: only the Leaflet renderer supports full pre-download (raster
+// tiles via sync/tilePrefetcher.ts). Mapbox GL is best-effort offline — its
+// vector tiles are cached opportunistically by the Service Worker as you view
+// them online (see the mapbox-tiles rule in vite.config.js), not prefetched.
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function MapViewAuto(props: any) {
   const provider = useSettingsStore(s => s.settings.map_provider)