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/tests/unit/sync/persistentStorage.test.ts

@@ -0,0 +1,47 @@
+/**
+ * requestPersistentStorage (H8 / M6) — best-effort persistent storage request
+ * so prefetched tiles / file blobs / IndexedDB aren't evicted under pressure.
+ */
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { requestPersistentStorage } from '../../../src/sync/persistentStorage';
+
+const original = (navigator as Navigator & { storage?: StorageManager }).storage;
+
+afterEach(() => {
+  Object.defineProperty(navigator, 'storage', { value: original, configurable: true });
+  vi.restoreAllMocks();
+});
+
+function stubStorage(storage: unknown) {
+  Object.defineProperty(navigator, 'storage', { value: storage, configurable: true });
+}
+
+describe('requestPersistentStorage', () => {
+  it('requests persistence when not already granted', async () => {
+    const persist = vi.fn().mockResolvedValue(true);
+    const persisted = vi.fn().mockResolvedValue(false);
+    stubStorage({ persist, persisted });
+
+    expect(await requestPersistentStorage()).toBe(true);
+    expect(persist).toHaveBeenCalledOnce();
+  });
+
+  it('skips the prompt when already persisted', async () => {
+    const persist = vi.fn().mockResolvedValue(true);
+    const persisted = vi.fn().mockResolvedValue(true);
+    stubStorage({ persist, persisted });
+
+    expect(await requestPersistentStorage()).toBe(true);
+    expect(persist).not.toHaveBeenCalled();
+  });
+
+  it('returns false (no throw) when the API is unavailable', async () => {
+    stubStorage(undefined);
+    expect(await requestPersistentStorage()).toBe(false);
+  });
+
+  it('returns false (no throw) when persist rejects', async () => {
+    stubStorage({ persist: vi.fn().mockRejectedValue(new Error('denied')) });
+    expect(await requestPersistentStorage()).toBe(false);
+  });
+});