Introduce "sources" util class

src/modules/source/services/Sources.ts

@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+import { SourceIdInfo } from '@/modules/source/Source.types.ts';
+
+export class Sources {
+    static readonly LOCAL_SOURCE_ID = '0';
+
+    static isLocalSource(source: SourceIdInfo): boolean {
+        return source.id === Sources.LOCAL_SOURCE_ID;
+    }
+}