add support for token validation on login page

lib/components/login/login.dart

@@ -1,10 +1,37 @@
 import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
 import 'package:mileograph_flutter/services/authservice.dart';
 import 'package:provider/provider.dart';
 
-class LoginScreen extends StatelessWidget {
+class LoginScreen extends StatefulWidget {
   const LoginScreen({super.key});
 
+  @override
+  State<LoginScreen> createState() => _LoginScreenState();
+}
+
+class _LoginScreenState extends State<LoginScreen> {
+  bool _checkingSession = true;
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addPostFrameCallback((_) => _checkExistingSession());
+  }
+
+  Future<void> _checkExistingSession() async {
+    final auth = context.read<AuthService>();
+    try {
+      final valid = await auth.validateStoredToken();
+      if (!valid) return;
+      await auth.tryRestoreSession();
+      if (!mounted) return;
+      context.go('/');
+    } finally {
+      if (mounted) setState(() => _checkingSession = false);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -24,7 +51,7 @@ class LoginScreen extends StatelessWidget {
                         color: Theme.of(context).textTheme.bodyLarge?.color,
                       ),
                     ),
-                    TextSpan(
+                    const TextSpan(
                       text: "O",
                       style: TextStyle(color: Colors.red),
                     ),
@@ -35,18 +62,27 @@ class LoginScreen extends StatelessWidget {
                       ),
                     ),
                   ],
-                  style: TextStyle(
+                  style: const TextStyle(
                     decoration: TextDecoration.none,
                     color: Colors.white,
-                  fontFamily: "Tomatoes",
-                  fontSize: 50,
+                    fontFamily: "Tomatoes",
+                    fontSize: 50,
+                  ),
                 ),
               ),
-            ),
-            const SizedBox(height: 50),
-            LoginPanel(),
-          ],
-        ),
+              if (_checkingSession)
+                const Padding(
+                  padding: EdgeInsets.only(top: 12),
+                  child: SizedBox(
+                    height: 24,
+                    width: 24,
+                    child: CircularProgressIndicator(strokeWidth: 2),
+                  ),
+                ),
+              const SizedBox(height: 50),
+              const LoginPanel(),
+            ],
+          ),
         ),
       ),
     );