refactor: update app initialization for new OAuth flow

Wire up VoteService with the new session getter and auth callbacks
for the backend-delegated OAuth architecture.

Key changes:
- VoteService now uses sessionGetter instead of direct OAuthSession
- Add tokenRefresher callback for automatic 401 recovery
- Add signOutHandler callback for failed refresh cleanup
- Remove OAuthService initialization (deleted)

The new flow ensures votes go through the Coves backend which
has the DPoP keys needed to write to user PDSs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

lib/main.dart

···
   final authProvider = AuthProvider();
   await authProvider.initialize();
 
-  // Initialize vote service with auth callbacks for direct PDS writes
-  // Uses DPoP authentication (not Bearer tokens!)
+  // Initialize vote service with auth callbacks
+  // Votes go through the Coves backend (which proxies to PDS with DPoP)
+  // Includes token refresh and sign-out handlers for automatic 401 recovery
   final voteService = VoteService(
     sessionGetter: () async => authProvider.session,
     didGetter: () => authProvider.did,
-    pdsUrlGetter: authProvider.getPdsUrl,
+    tokenRefresher: authProvider.refreshToken,
+    signOutHandler: authProvider.signOut,
   );
 
   runApp(