fix: fall back to regular OIDC when silent auth fails

When prompt=none fails with login_required (no Authentik session),
automatically redirect to regular OIDC flow to show login UI.

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

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

lib/routing/app_router.dart

@@ -137,6 +137,13 @@ class _OidcCallbackPageState extends ConsumerState<_OidcCallbackPage> {
 
     // Check for error from Authentik
     if (widget.error != null) {
+      // Silent OIDC (prompt=none) failed - no existing session
+      // Fall back to regular OIDC flow to show login UI
+      if (widget.error == 'login_required') {
+        ref.read(authProvider.notifier).signIn();
+        return;
+      }
+
       setState(() {
         _isProcessing = false;
         _error = widget.errorDescription ?? widget.error;