fix: allow callback route to complete Auth0 code exchange (refs #189)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -557,8 +557,8 @@ function App() {
     );
   }
 
-  // Callback route requires authentication - handled by CallbackPage component
-  if (isCallbackRoute && isAuthenticated) {
+  if (isCallbackRoute) {
+    if (isAuthenticated) {
       return (
         <ThemeProvider>
           <React.Suspense fallback={
@@ -572,6 +572,25 @@ function App() {
         </ThemeProvider>
       );
     }
+    if (mobileMode) {
+      return (
+        <ThemeProvider>
+          <Layout mobileMode={true}>
+            <div className="flex items-center justify-center h-64">
+              <div className="text-slate-500">Processing login...</div>
+            </div>
+          </Layout>
+        </ThemeProvider>
+      );
+    }
+    return (
+      <ThemeProvider>
+        <div className="flex items-center justify-center min-h-screen">
+          <div className="text-lg">Processing login...</div>
+        </div>
+      </ThemeProvider>
+    );
+  }
 
   if (shouldShowHomePage) {
     return (