feat(frontend): implement category list, create, and edit pages

- /categories: list with colored pills, search + clear, sort/order controls
- /categories/new: create form with name, color picker, notes, is_public
- /categories/[id]: edit form + tags-in-category section with load more
- sorting.ts: add categorySorting store (name/color/created, persisted)
- mock: category CRUD, GET /categories/{id}/tags, search/sort/offset

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/stores/sorting.ts

@@ -41,4 +41,11 @@ export const fileSorting = makeSortStore<FileSortField>('sort:files', {
 export const tagSorting = makeSortStore<TagSortField>('sort:tags', {
 	sort: 'created',
 	order: 'desc',
+});
+
+export type CategorySortField = 'name' | 'color' | 'created';
+
+export const categorySorting = makeSortStore<CategorySortField>('sort:categories', {
+	sort: 'name',
+	order: 'asc',
 });