Fix stale logged-in UI when auth verify fails on network error

[nimra06]

Description

After a successful login, the app stores bibliodrift_user and isLoggedIn in local storage and runs verifyStoredAuthSession() on page load to validate the current session through GET /api/v1/auth/verify.

Previously, if the verification request failed because the backend was unreachable (backend stopped, offline mode, CORS issue, wrong origin, etc.), the app would still fall back to the cached storedUser from local storage. This caused the navbar to continue showing logged-in UI even though the session was never verified.

This PR updates verifyStoredAuthSession() in frontend/js/app.js so failed verification requests are treated the same as invalid sessions.

What changed

• Clear stored auth data when session verification fails
• Return null instead of using cached session state
• Show signed-out navigation until verification succeeds again
• Keep behavior consistent with existing 401 / 422 handling
• Remove the duplicate credentials: 'include' entry from the same fetch call

Related Issue

Fixes #730

Type of Change

• Bug Fix
• Feature
• Enhancement
• Documentation

Testing

1. Serve the frontend over HTTP
2. Start the Flask backend
3. Register or log in with a real account (not demo@bibliodrift.com)
4. Confirm:
   • Logged-in navigation is visible
   • bibliodrift_user and isLoggedIn exist in local storage
5. Stop the backend (or enable DevTools → Network → Offline)
6. Reload index.html or library.html

Expected Result

• Navbar shows signed-out navigation
• Local auth keys are cleared
• Console warns that session verification failed
• Cached session state is no longer used

7. Restart the backend and reload the page
8. User should be required to sign in again

Screenshots

Optional: before/after navbar state when backend is unavailable.

Checklist

• Code follows project guidelines
• Tested locally
• Documentation updated (not required)
• PR title includes the appropriate contribution tag

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[Wahid7852]

Looks good to me overall. Only thing worth noting is that temporary backend/network issues will now force logout instead of keeping cached auth state.