D1APP-43 username validation added

lib/pages/authentication/screens/username.dart

@@ -6,17 +6,25 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
-class UsernameInput extends StatelessWidget {
+class UsernameInput extends StatefulWidget {
   const UsernameInput({
     Key? key,
   }) : super(key: key);
 
+  @override
+  State<UsernameInput> createState() => _UsernameInputState();
+}
+
+class _UsernameInputState extends State<UsernameInput> {
+  final _formKey = GlobalKey<FormState>();
   @override
   Widget build(BuildContext context) {
     final AuthenticationState state = context.read<AuthenticationState>();
     return AuthenticationLayout(
       children: [
-        DidvanTextField(
+        Form(
+          key: _formKey,
+          child: DidvanTextField(
             initialValue: state.username,
             title: 'نام کاربری یا شماره موبایل',
             hintText: 'نام کاربری یا شماره موبایل',
@@ -28,12 +36,17 @@ class UsernameInput extends StatelessWidget {
               state.username = value;
             },
           ),
+        ),
         const SizedBox(
           height: 20,
         ),
         DidvanButton(
           title: 'ورود',
-          onPressed: state.confirmUsername,
+          onPressed: () {
+            if (_formKey.currentState!.validate()) {
+              state.confirmUsername();
+            }
+          },
         ),
         const Spacer(),
         Padding(