Make appId Assignable only once

lib/pages/add_app.dart

@@ -538,6 +538,22 @@ class AddAppPageState extends State<AddAppPage> {
                       });
                     }
                   }),
+            if (pickedSource != null)
+              GeneratedForm(
+                  key: const Key('appId'),
+                  items: [
+                    [
+                      GeneratedFormTextField('appId',
+                          label: tr('appId'), required: false),
+                    ]
+                  ],
+                  onValueChanges: (values, valid, isBuilding) {
+                    if (!isBuilding) {
+                      setState(() {
+                        additionalSettings['appId'] = values['appId'];
+                      });
+                    }
+                  }),
           ],
         );
 

lib/providers/source_provider.dart

@@ -572,7 +572,6 @@ abstract class AppSource {
           label: tr('skipUpdateNotifications'))
     ],
     [GeneratedFormTextField('about', label: tr('about'), required: false)],
-    [GeneratedFormTextField('appId', label: tr('appId'), required: false)]
   ];
 
   // Previous 2 variables combined into one at runtime for convenient usage