fix: skip silent OIDC on callback page to prevent race condition

AuthProvider.build() was initiating silent OIDC while the callback
page was processing the auth code, causing PKCE state to be cleared.
Now checks if on /callback route and skips silent OIDC initiation.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/core/auth/auth_provider.dart

@@ -42,6 +42,13 @@ class AuthNotifier extends _$AuthNotifier {
   Future<AuthState> build() async {
     // On web with auth required, use silent OIDC to get JWT
     if (kIsWeb && AppConfig.requiresAuth) {
+      // Skip silent OIDC if we're on the callback page (it will handle auth)
+      final currentUrl = web_utils.getCurrentUrl();
+      if (currentUrl.contains('/callback')) {
+        developer.log('Web: On callback page, skipping silent OIDC', name: 'auth');
+        return const AuthState();
+      }
+
       // First check if we have stored tokens
       final storedAuth = await _loadStoredAuth();
       if (storedAuth.isAuthenticated && !storedAuth.isTokenExpired) {