fix(pwa): fix offline session redirect and file download auth (#505 #541)

**#541 — File downloads broken in PWA standalone mode**
Replace getAuthUrl + window.open pattern with blob-based fetch using
credentials:include. The old approach minted a 60s single-use ephemeral
token then called window.open, which handed the URL to the system browser
on Android/iOS — losing the PWA cookie jar and producing "invalid or
expired token". The new approach fetches the file directly inside the
PWA WebView as a blob URL, so no auth handoff occurs.

New helper client/src/utils/fileDownload.ts with downloadFile and openFile.
Updated FileManager, ReservationsPanel, ReservationModal, PlaceInspector,
CollabNotes.

Security hardening in fileDownload.ts:
- assertRelativeUrl() guard prevents credentials being sent to external hosts
- openFile() checks blob.type against a safe-inline allowlist; HTML, SVG and
  other script-capable MIME types are forced to download instead of being
  opened inline, preventing same-origin XSS via blob URLs
- resp.ok check covers all non-2xx responses, not just 401

**#505 — PWA offline session lost on reload**
Wrap authStore with Zustand persist middleware, serializing only
{user, isAuthenticated} to localStorage key trek_auth_snapshot.
maps_api_key is intentionally excluded from the snapshot.

On cold start with no network: persist hydrates isAuthenticated:true,
App.tsx clears isLoading and calls loadUser({silent:true}), ProtectedRoute
renders the dashboard immediately. The network error from loadUser leaves
isAuthenticated intact so no login redirect occurs.

On 401 or logout: store state is cleared, persist writes
{isAuthenticated:false} — stale snapshot does not grant offline access
after session expiry.

client/src/App.tsx

@@ -93,7 +93,15 @@ export default function App() {
 
   useEffect(() => {
     if (!location.pathname.startsWith('/shared/') && !location.pathname.startsWith('/public/') && !location.pathname.startsWith('/login')) {
-      loadUser()
+      // If the persist snapshot already has an authenticated user, validate
+      // silently so the PWA shell renders immediately without a spinner.
+      const alreadyAuthenticated = useAuthStore.getState().isAuthenticated
+      if (alreadyAuthenticated) {
+        useAuthStore.setState({ isLoading: false })
+        loadUser({ silent: true })
+      } else {
+        loadUser()
+      }
     }
     authApi.getAppConfig().then(async (config: { demo_mode?: boolean; dev_mode?: boolean; is_prerelease?: boolean; has_maps_key?: boolean; version?: string; timezone?: string; require_mfa?: boolean; trip_reminders_enabled?: boolean; permissions?: Record<string, PermissionLevel> }) => {
       if (config?.demo_mode) setDemoMode(true)