feat: 新增桌面应用支持

- 新增 desktop 应用入口，将后端与前端打包为单一可执行文件
- 集成系统托盘功能（getlantern/systray）
- 支持单实例锁和端口冲突检测
- 启动时自动打开浏览器显示管理界面
- 新增 embedfs 模块嵌入静态资源
- 新增跨平台构建脚本（macOS/Windows/Linux）
- 新增 macOS .app 打包脚本
- 统一 Makefile，移除 backend/Makefile
- 更新 README 添加桌面应用使用说明

backend/go.mod

@@ -2,12 +2,15 @@ module nex/backend
 
 go 1.26.2
 
+replace nex/embedfs => ../embedfs
+
 tool (
 	github.com/golangci/golangci-lint/cmd/golangci-lint
 	go.uber.org/mock/mockgen
 )
 
 require (
+	github.com/getlantern/systray v1.2.2
 	github.com/gin-gonic/gin v1.12.0
 	github.com/go-playground/validator/v10 v10.30.2
 	github.com/google/uuid v1.6.0
@@ -22,6 +25,7 @@ require (
 	gopkg.in/yaml.v3 v3.0.1
 	gorm.io/driver/sqlite v1.6.0
 	gorm.io/gorm v1.31.1
+	nex/embedfs v0.0.0-00010101000000-000000000000
 )
 
 require (
@@ -74,11 +78,18 @@ require (
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/fzipp/gocyclo v0.6.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.13 // indirect
+	github.com/getlantern/context v0.0.0-20190109183933-c447772a6520 // indirect
+	github.com/getlantern/errors v0.0.0-20190325191628-abdb3e3e36f7 // indirect
+	github.com/getlantern/golog v0.0.0-20190830074920-4ef2e798c2d7 // indirect
+	github.com/getlantern/hex v0.0.0-20190417191902-c6586a6fe0b7 // indirect
+	github.com/getlantern/hidden v0.0.0-20190325191715-f02dbb02be55 // indirect
+	github.com/getlantern/ops v0.0.0-20190325191751-d70cb0d6f85f // indirect
 	github.com/ghostiam/protogetter v0.3.9 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
 	github.com/go-critic/go-critic v0.12.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
+	github.com/go-stack/stack v1.8.0 // indirect
 	github.com/go-toolsmith/astcast v1.1.0 // indirect
 	github.com/go-toolsmith/astcopy v1.1.0 // indirect
 	github.com/go-toolsmith/astequal v1.2.0 // indirect
@@ -153,6 +164,7 @@ require (
 	github.com/nishanths/predeclared v0.2.2 // indirect
 	github.com/nunnatsa/ginkgolinter v0.19.1 // indirect
 	github.com/olekukonko/tablewriter v0.0.5 // indirect
+	github.com/oxtoacart/bpool v0.0.0-20190530202638-03653db5a59c // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/polyfloyd/go-errorlint v1.7.1 // indirect