feat: collapsible hierarchical category tags + category sort for shelf

- Backend: add parent_category param (matches '历史-*' sub-categories),
  sort=category mode (ORDER BY category ASC), return category_tree with
  parent→sub→count hierarchy
- Frontend: replace flat 63-tag list with collapsible parent tags showing
  book counts, click parent to expand sub-category pills with fade-in
  animation, click sub to filter to exact category
- Add '分类排序' toggle button: when active, books are grouped by
  category with colored section headers showing category name + count
- Clicking same parent again collapses it; '全部' resets everything
- Mobile responsive: smaller tag sizes, reduced indent on narrow screens

templates/index.html

@@ -125,6 +125,7 @@
                         <label class="filter-checkbox">
                             <input type="checkbox" id="finishedOnly"> 仅看已读完
                         </label>
+                        <button class="shelf-sort-btn" id="shelfSortBtn" onclick="toggleShelfSort()">分类排序</button>
                     </div>
                     <div class="shelf-info" id="shelfInfo"></div>
                 </div>