fix: update callback route and enhance user profile link in header

- Changed the OAuth callback route from `/auth/callback` to `/callback` in App component and .env.example.
- Added user profile link in AppHeader for authenticated users, directing to their dashboard.
- Removed bypass mode references from LandingPage to streamline the login flow.
- Introduced a utility function to extract ORCID from JWT in AuthContext for better user state management.

frontend/src/services/api.js

@@ -145,7 +145,6 @@ function normalizePublication(p) {
     hash_fingerprint: p.hash_fingerprint ?? null,
     last_modified: p.last_modified ?? null,
     status: p.status ?? null,
-    // null when request was made without a JWT (user not logged in)
     downloaded_by_me: p.downloaded_by_me ?? null,
   };
 }