fix(auth): defer OIDC callback processing to post-frame

Wraps _processCallback() in addPostFrameCallback to avoid Riverpod
"Tried to modify a provider while the widget tree was building" error.

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

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

lib/routing/app_router.dart

@@ -286,7 +286,10 @@ class _OidcCallbackPageState extends ConsumerState<_OidcCallbackPage> {
   @override
   void initState() {
     super.initState();
-    _processCallback();
+    // Defer callback processing to avoid Riverpod state modification during build
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      _processCallback();
+    });
   }
 
   Future<void> _processCallback() async {