Fix library gql query caching issue

With 8b8da30335 the library now calls `useGetMigratableSources` which uses the same `mangas` query as `GET_LIBRARY_MANGA_COUNT`.
However, they select different fields, which causes the cache to get overwritte, because it apollo-client wasn't merging the results.

src/lib/requests/client/GraphQLClient.ts

@@ -136,6 +136,12 @@ const typePolicies: TypedTypePolicies = {
             lastSyncStatus(_, { toReference }) {
                 return toReference({ __typename: 'SyncStatus', key: {} });
             },
+            mangas: {
+                keyArgs: ['condition', 'filter', 'orderBy', 'orderByType', 'order'],
+                merge(existing, incoming, { mergeObjects }) {
+                    return mergeObjects(existing, incoming);
+                },
+            },
             chapters: {
                 keyArgs: ['condition', 'filter', 'orderBy', 'orderByType', 'order'],
                 merge(existing, incoming) {