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/App.jsx

@@ -19,7 +19,7 @@ export default function App() {
         <Route path="/" element={<LandingPage />} />
         <Route path="/dashboard/:orcid" element={<DashboardPage />} />
         <Route path="/group" element={<GroupResultsPage />} />
-        <Route path="/auth/callback" element={<AuthCallbackPage />} />
+        <Route path="/callback" element={<AuthCallbackPage />} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>