fix Subsonic root folder condition (#1686)

- some older subsonic servers used numeric ids which causes the rootFolderId regex to fail which resulted in the getFolder endpoint to always attempt to fetch the root

src/renderer/api/subsonic/subsonic-controller.ts

@@ -763,7 +763,7 @@ export const SubsonicController: InternalControllerEndpoint = {
     getFolder: async ({ apiClientProps, context, query }) => {
         const sortOrder = (query.sortOrder?.toLowerCase() ?? 'asc') as 'asc' | 'desc';
 
-        const isRootFolderId = /^\d+$/.test(query.id);
+        const isRootFolderId = query.id === '0';
 
         if (isRootFolderId) {
             const res = await ssApiClient(apiClientProps).getIndexes({