mobile: Allow account deletion

mobile/lib/screens/account_settings.dart

@@ -42,12 +42,16 @@ class _AccountSettingsScreenState extends State<AccountSettingsScreen> {
   }
 
   Future<void> _setCredentials(MusicusAccountCredentials credentials) async {
-    if (credentials != null) {
     if (mounted) {
+      if (credentials != null) {
         setState(() {
           _loggedIn = true;
           _username = credentials.username;
         });
+      } else {
+        setState(() {
+          _loggedIn = false;
+        });
       }
     }
   }
@@ -108,6 +112,36 @@ class _AccountSettingsScreenState extends State<AccountSettingsScreen> {
             );
           },
         ),
+        ListTile(
+          title: Text('Delete this account'),
+          onTap: () async {
+            showDialog(
+              context: context,
+              builder: (context) => AlertDialog(
+                title: Text('Delete account'),
+                content: Text(
+                    'Do you really want to delete the account with the '
+                    'username $_username?'),
+                actions: <Widget>[
+                  FlatButton(
+                    child: Text('DELETE'),
+                    onPressed: () async {
+                      await _backend.client.deleteAccount();
+                      await _backend.settings.clearAccount();
+                      Navigator.pop(context);
+                    },
+                  ),
+                  FlatButton(
+                    child: Text('CANCEL'),
+                    onPressed: () {
+                      Navigator.pop(context);
+                    },
+                  ),
+                ],
+              ),
+            );
+          },
+        ),
         ListTile(
           title: Text('Logout'),
           onTap: () async {