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/cmd/desktop/static_test.go

@@ -0,0 +1,123 @@
+package main
+
+import (
+	"io/fs"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+
+	"nex/embedfs"
+)
+
+func TestSetupStaticFiles(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	distFS, err := fs.Sub(embedfs.FrontendDist, "frontend-dist")
+	if err != nil {
+		t.Skipf("跳过测试: 前端资源未构建: %v", err)
+		return
+	}
+
+	getContentType := func(path string) string {
+		if strings.HasSuffix(path, ".js") {
+			return "application/javascript"
+		}
+		if strings.HasSuffix(path, ".css") {
+			return "text/css"
+		}
+		if strings.HasSuffix(path, ".svg") {
+			return "image/svg+xml"
+		}
+		return "application/octet-stream"
+	}
+
+	r := gin.New()
+	r.GET("/assets/*filepath", func(c *gin.Context) {
+		filepath := c.Param("filepath")
+		data, err := fs.ReadFile(distFS, "assets"+filepath)
+		if err != nil {
+			c.Status(404)
+			return
+		}
+		c.Data(200, getContentType(filepath), data)
+	})
+
+	r.GET("/favicon.svg", func(c *gin.Context) {
+		data, err := fs.ReadFile(distFS, "favicon.svg")
+		if err != nil {
+			c.Status(404)
+			return
+		}
+		c.Data(200, "image/svg+xml", data)
+	})
+
+	r.NoRoute(func(c *gin.Context) {
+		path := c.Request.URL.Path
+		if strings.HasPrefix(path, "/api/") ||
+			strings.HasPrefix(path, "/v1/") ||
+			strings.HasPrefix(path, "/health") {
+			c.JSON(404, gin.H{"error": "not found"})
+			return
+		}
+		data, err := fs.ReadFile(distFS, "index.html")
+		if err != nil {
+			c.Status(500)
+			return
+		}
+		c.Data(200, "text/html; charset=utf-8", data)
+	})
+
+	t.Run("API 404", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/api/test", nil)
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Code != 404 {
+			t.Errorf("期望状态码 404, 实际 %d", w.Code)
+		}
+	})
+
+	t.Run("SPA fallback", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/providers", nil)
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Code != 200 {
+			t.Errorf("期望状态码 200, 实际 %d", w.Code)
+		}
+	})
+
+	t.Run("MIME type for JS", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/assets/test.js", nil)
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Code == 200 {
+			expected := "application/javascript"
+			if w.Header().Get("Content-Type") != expected {
+				t.Errorf("期望 Content-Type %s, 实际 %s", expected, w.Header().Get("Content-Type"))
+			}
+		} else {
+			t.Log("文件不存在，跳过 MIME 类型验证")
+		}
+	})
+
+	t.Run("MIME type for CSS", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/assets/test.css", nil)
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Code == 200 {
+			expected := "text/css"
+			if w.Header().Get("Content-Type") != expected {
+				t.Errorf("期望 Content-Type %s, 实际 %s", expected, w.Header().Get("Content-Type"))
+			}
+		} else {
+			t.Log("文件不存在，跳过 MIME 类型验证")
+		}
+	})
+
+	t.Log("静态文件服务测试通过")
+}