书架恢复横向网格展示，移除分类分组标题

- loadShelf 始终用简单网格渲染，不再调用 renderBooksGroupedByCategory
- 删除 renderBooksGroupedByCategory 函数
- 清理 .shelf-group-header / .shelf-group-count CSS 样式
- 后端 sort=category 仍生效（按分类排序），前端不显示分组标题
- 分类筛选标签保留

static/js/app.js

@@ -690,14 +690,7 @@ async function loadShelf(page) {
         document.getElementById("shelfInfo").textContent =
             `共 ${data.total} 本 · 第 ${page}/${data.total_pages} 页`;
 
-        if (shelfSortMode === "category" && data.books.length > 0) {
-            // 按分类分组渲染
-            grid.innerHTML = renderBooksGroupedByCategory(data.books);
-        } else {
-            grid.innerHTML = data.books
-                .map((book) => renderBookCard(book))
-                .join("");
-        }
+        grid.innerHTML = data.books.map((book) => renderBookCard(book)).join("");
 
         renderPagination(page, data.total_pages);
     } catch (e) {
@@ -722,23 +715,6 @@ function renderBookCard(book) {
     </div>`;
 }
 
-function renderBooksGroupedByCategory(books) {
-    // 按 category 分组
-    const groups = {};
-    books.forEach((book) => {
-        const cat = book.category || "未分类";
-        if (!groups[cat]) groups[cat] = [];
-        groups[cat].push(book);
-    });
-
-    let html = "";
-    for (const [cat, items] of Object.entries(groups)) {
-        html += `<div class="shelf-group-header">${cat} <span class="shelf-group-count">${items.length} 本</span></div>`;
-        html += `<div class="book-grid">${items.map(renderBookCard).join("")}</div>`;
-    }
-    return html;
-}
-
 function renderPagination(current, total) {
     const pag = document.getElementById("pagination");
     if (total <= 1) { pag.innerHTML = ""; return; }