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/sync/persistentStorage.ts

@@ -0,0 +1,18 @@
+/**
+ * Ask the browser for persistent storage so our offline data — prefetched map
+ * tiles, cached file blobs, the IndexedDB caches — is exempt from eviction under
+ * storage pressure. Without this the browser may purge tiles right when a
+ * traveler goes offline and needs them (audit H8 / M6).
+ *
+ * Best-effort and idempotent: returns whether persistence is (now) granted.
+ */
+export async function requestPersistentStorage(): Promise<boolean> {
+  try {
+    if (typeof navigator === 'undefined' || !navigator.storage?.persist) return false
+    // Already persisted? Avoid re-prompting where the API distinguishes.
+    if (navigator.storage.persisted && (await navigator.storage.persisted())) return true
+    return await navigator.storage.persist()
+  } catch {
+    return false
+  }
+}

client/src/sync/tilePrefetcher.ts

@@ -189,8 +189,11 @@ export async function prefetchTilesForTrip(
   // their lower zooms cached instead of being skipped entirely.
   //
   // NOTE: opaque (no-cors) tile responses are padded by Chromium to ~7 MB each
-  // for quota accounting, so the real on-disk budget is far below 180 MB. That
-  // (audit H8) and navigator.storage.persist() (M6) are tracked separately.
+  // for quota accounting, so the real on-disk budget is far below 180 MB. We
+  // keep no-cors deliberately: switching to cors would break self-hosted/custom
+  // tile providers that don't send CORS headers. To stop the browser evicting
+  // these tiles under the inflated quota, we request persistent storage at app
+  // init instead (sync/persistentStorage.ts).
   const fetched = await prefetchTiles(bbox, template)
 
   // Update syncMeta with bbox and tile count