feat(catalog): add reply-count sorting mode

src/stores/use-sorting-store.ts

@@ -1,8 +1,11 @@
 import { create } from 'zustand';
 
+/** Catalog-only sort state. Used by catalog view sort selector. */
+export type CatalogSortType = 'active' | 'new' | 'replyCount';
+
 interface SortingStore {
-  sortType: 'active' | 'new';
-  setSortType: (type: 'active' | 'new') => void;
+  sortType: CatalogSortType;
+  setSortType: (type: CatalogSortType) => void;
 }
 
 const useSortingStore = create<SortingStore>((set) => ({