新增：Markdown编辑器/数据库优化/安全修复

- Markdown 编辑器：实时预览、PDF 导出、独立查看器
- 数据库优化：动态连接池、查询缓存、Redis Pipeline
- 窗口置顶功能
- 文件系统增强：右键菜单、编辑器集成、收藏夹重构
- 安全修复：XSS 防护、路径穿越、HTML 注入
- 代码质量：正则预编译、缓存锁优化、死代码清理

go.mod

@@ -3,11 +3,15 @@ module u-desk
 go 1.25.6
 
 require (
+	github.com/chromedp/cdproto v0.0.0-20250724212937-08a3db8b4327
+	github.com/chromedp/chromedp v0.14.2
 	github.com/glebarez/sqlite v1.11.0
 	github.com/go-sql-driver/mysql v1.9.3
+	github.com/jung-kurt/gofpdf v1.16.2
 	github.com/redis/go-redis/v9 v9.17.3
 	github.com/shirou/gopsutil/v3 v3.24.5
 	github.com/wailsapp/wails/v2 v2.11.0
+	github.com/yuin/goldmark v1.8.2
 	go.mongodb.org/mongo-driver/v2 v2.5.0
 	gorm.io/driver/mysql v1.6.0
 	gorm.io/gorm v1.31.1
@@ -17,10 +21,15 @@ require (
 	filippo.io/edwards25519 v1.1.0 // indirect
 	github.com/bep/debounce v1.2.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/chromedp/sysutil v1.1.0 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/glebarez/go-sqlite v1.22.0 // indirect
+	github.com/go-json-experiment/json v0.0.0-20250725192818-e39067aee2d2 // indirect
 	github.com/go-ole/go-ole v1.3.0 // indirect
+	github.com/gobwas/httphead v0.1.0 // indirect
+	github.com/gobwas/pool v0.2.1 // indirect
+	github.com/gobwas/ws v1.4.0 // indirect
 	github.com/godbus/dbus/v5 v5.2.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect