fix: 桌面应用跨平台编译和单实例锁

- 使用 gofrs/flock 替代 syscall.Flock 以支持 Windows
- 引入 SingletonLock 结构体，支持锁路径参数化（测试与生产隔离）
- 对齐服务初始化流程与 cmd/server（RoutingCache、StatsBuffer）
- 添加 gofrs/flock 依赖
- 重写单例测试，覆盖加锁/解锁/重复加锁场景
- 更新 desktop-app 规范，补充跨平台锁细节
- 新增 cross-platform-singleton 规范

backend/go.mod

@@ -13,6 +13,7 @@ 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/gofrs/flock v0.13.0
 	github.com/google/uuid v1.6.0
 	github.com/mitchellh/mapstructure v1.5.0
 	github.com/pressly/goose/v3 v3.27.0
@@ -102,7 +103,6 @@ require (
 	github.com/gobwas/glob v0.2.3 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
-	github.com/gofrs/flock v0.12.1 // indirect
 	github.com/golang/protobuf v1.5.3 // indirect
 	github.com/golangci/dupl v0.0.0-20250308024227-f665c8d69b32 // indirect
 	github.com/golangci/go-printf-func-name v0.1.0 // indirect