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/handler/overview.go

@@ -29,20 +29,20 @@ func (h *Handlers) Overview(c *gin.Context) {
 		}
 	}
 	c.JSON(http.StatusOK, gin.H{
-		"stats":               data.Stats,
-		"hot":                 data.Hot,
-		"active_users":        data.ActiveUsers,
-		"boards":              data.Boards,
-		"announcements":       data.Announcements,
-		"announcements_total": data.AnnouncementsTotal,
-		"sidebar_pages":       data.SidebarPages,
-		"new_users":           data.NewUsers,
-		"checkin":             data.Checkin,
-		"ads":                 data.Ads,
-		"ads_panel_title":     data.AdsPanelTitle,
-		"ads_enabled":         data.AdsEnabled,
-		"sponsors":            data.Sponsors,
+		"stats":                data.Stats,
+		"hot":                  data.Hot,
+		"active_users":         data.ActiveUsers,
+		"boards":               data.Boards,
+		"announcements":        data.Announcements,
+		"announcements_total":  data.AnnouncementsTotal,
+		"sidebar_pages":        data.SidebarPages,
+		"new_users":            data.NewUsers,
+		"checkin":              data.Checkin,
+		"ads":                  data.Ads,
+		"ads_panel_title":      data.AdsPanelTitle,
+		"ads_enabled":          data.AdsEnabled,
+		"sponsors":             data.Sponsors,
 		"sponsors_panel_title": data.SponsorsPanelTitle,
-		"sponsors_enabled":    data.SponsorsEnabled,
+		"sponsors_enabled":     data.SponsorsEnabled,
 	})
 }