feat: configurable menu bar position for mailbox view (#298) (#303)

lib/core/db_schema_version.dart

@@ -1 +1 @@
-const int dbSchemaVersion = 33;
+const int dbSchemaVersion = 34;

lib/core/models/user_preferences.dart

@@ -0,0 +1,6 @@
+enum MenuPosition { bottom, top }
+
+class UserPreferences {
+  const UserPreferences({this.menuPosition = MenuPosition.bottom});
+  final MenuPosition menuPosition;
+}

lib/core/repositories/user_preferences_repository.dart

@@ -0,0 +1,6 @@
+import 'package:sharedinbox/core/models/user_preferences.dart';
+
+abstract class UserPreferencesRepository {
+  Stream<UserPreferences> observePreferences();
+  Future<void> updateMenuPosition(MenuPosition position);
+}