feat: 书库导入导出/图片变体/书籍搜索/小组件运行时等

新增：
- 书库导入导出（library_import/library_export）及测试
- 图片变体生成（image_variants）与响应式图片（responsiveImage）
- 书籍搜索（bookSearch）+ BookSearch/LibrarySearchGrid 组件
- 小组件运行时（widgetRuntime）与静态检查（widgetLint）
- 上传缓存中间件（upload_cache）与字体 CSS 提取脚本

其它：
- 后端 handlers/services 全量调整
- 前端页面、组件、库函数与配置更新

backend/service/media_library_test.go

@@ -250,8 +250,7 @@ func TestAdminMediaLibraryAttachSources(t *testing.T) {
 	if !ok {
 		t.Fatal("comment image missing")
 	}
-	if cimg.SourceURL != fmt.Sprintf("/post/%d#comment-", post.ID) &&
-		!strings.HasPrefix(cimg.SourceURL, fmt.Sprintf("/post/%d#comment-", post.ID)) {
+	if cimg.SourceURL != fmt.Sprintf("/post/%d#comment-1", post.ID) {
 		t.Fatalf("comment source url wrong: %+v", cimg)
 	}
 	if !strings.Contains(cimg.SourceLabel, "下的评论") {
@@ -259,6 +258,63 @@ func TestAdminMediaLibraryAttachSources(t *testing.T) {
 	}
 }
 
+func TestAdminMediaLibraryBookCategory(t *testing.T) {
+	s, dir := newMediaLibraryService(t)
+
+	// 书籍封面（本地磁盘）与正文插图（远程对象），另有一张普通帖子插图
+	writeMediaFile(t, dir, "images/cover.webp", []byte("fake-webp-bytes"))
+
+	if err := s.db.Create(&model.User{Username: "alice", Nickname: "爱丽丝"}).Error; err != nil {
+		t.Fatalf("create user: %v", err)
+	}
+	var alice model.User
+	if err := s.db.Where("username = ?", "alice").First(&alice).Error; err != nil {
+		t.Fatalf("load user: %v", err)
+	}
+	created := time.Date(2026, 9, 1, 10, 0, 0, 0, time.UTC)
+	mkAtt := func(source, url string) {
+		if err := s.db.Create(&model.Attachment{
+			UserID: alice.ID, Kind: model.AttachmentKindImage, Source: source,
+			URL: url, MIME: "image/webp", Size: 100, Width: 10, Height: 10,
+			CreatedAt: created,
+		}).Error; err != nil {
+			t.Fatalf("create attachment %s: %v", source, err)
+		}
+	}
+	mkAtt(model.AttachmentSourceLibraryCover, "/uploads/images/cover.webp")
+	mkAtt(model.AttachmentSourceLibraryContent, "/api/media/book-content")
+	mkAtt(model.AttachmentSourcePost, "/api/media/post-image")
+
+	items, counts, err := s.AdminMediaLibrary()
+	if err != nil {
+		t.Fatalf("AdminMediaLibrary: %v", err)
+	}
+	byURL := map[string]MediaLibraryItem{}
+	for _, it := range items {
+		byURL[it.URL] = it
+	}
+
+	// 封面（磁盘）与正文插图（远程）都归“书籍”
+	for _, u := range []string{"/uploads/images/cover.webp", "/api/media/book-content"} {
+		it, ok := byURL[u]
+		if !ok {
+			t.Fatalf("%s missing", u)
+		}
+		if it.Category != "book" || it.CategoryName != "书籍" {
+			t.Fatalf("%s should be book category, got %q/%q", u, it.Category, it.CategoryName)
+		}
+	}
+
+	// 普通帖子插图仍归 image
+	if post := byURL["/api/media/post-image"]; post.Category != "image" {
+		t.Fatalf("post image should stay image category, got %q", post.Category)
+	}
+
+	if counts["book"] != 2 || counts["image"] != 1 {
+		t.Fatalf("counts wrong: %v", counts)
+	}
+}
+
 func TestMediaThumb(t *testing.T) {
 	s, dir := newMediaLibraryService(t)