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/vite.config.js

@@ -47,6 +47,22 @@ export default defineConfig({
               cacheableResponse: { statuses: [0, 200] },
             },
           },
+          {
+            // Mapbox GL style, glyphs, sprites and vector tiles. Best-effort
+            // offline only: opportunistically caches what the user has already
+            // viewed online. Full pre-download offline maps require the Leaflet
+            // renderer (raster prefetch in tilePrefetcher.ts) — the GL vector
+            // pipeline is not prefetched. StaleWhileRevalidate keeps the basemap
+            // fresh online while still serving from cache when offline. Mapbox
+            // sends CORS, so responses are non-opaque (real 200s, no quota pad).
+            urlPattern: /^https:\/\/(api\.mapbox\.com|[a-d]\.tiles\.mapbox\.com)\/.*/i,
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'mapbox-tiles',
+              expiration: { maxEntries: 3000, maxAgeSeconds: 30 * 24 * 60 * 60 },
+              cacheableResponse: { statuses: [200] },
+            },
+          },
           {
             // API calls — network only. We deliberately do NOT cache API
             // responses in the Service Worker: Workbox keys entries by URL and