Set keyboard type for login form to emailAddress

lib/screens/sign_in.dart

@@ -50,6 +50,7 @@ class _SignInScreenState extends State<SignInScreen> {
                 TextFormField(
                   autovalidateMode: AutovalidateMode.onUserInteraction,
                   controller: usernameController,
+                  keyboardType: TextInputType.emailAddress,
                   validator: (value) => EmailValidator.validate(value ?? '')
                       ? null
                       : 'Not a valid Friendica Account Address',