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/cmd/desktop/singleton_test.go

@@ -3,37 +3,56 @@ package main
 import (
 	"os"
 	"path/filepath"
-	"syscall"
 	"testing"
 )
 
-func TestAcquireSingleInstance(t *testing.T) {
-	lockPath := filepath.Join(os.TempDir(), "nex-gateway-test.lock")
-	
-	origLockFile := lockFile
-	lockFile = nil
-	defer func() { lockFile = origLockFile }()
-
-	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0666)
-	if err != nil {
-		t.Fatalf("无法创建锁文件: %v", err)
-	}
-	defer f.Close()
+func TestSingletonLock_FirstLockSuccess(t *testing.T) {
+	lockPath := filepath.Join(os.TempDir(), "nex-gateway-test-first.lock")
 	defer os.Remove(lockPath)
 
-	err = syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
-	if err != nil {
-		t.Fatalf("无法获取文件锁: %v", err)
+	lock := NewSingletonLock(lockPath)
+	if err := lock.Lock(); err != nil {
+		t.Fatalf("首次加锁应成功，但返回错误: %v", err)
 	}
-	defer syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
-
-	t.Log("单实例锁测试通过")
+	defer lock.Unlock()
 }
 
-func TestReleaseSingleInstance(t *testing.T) {
-	lockFile = nil
-	
-	releaseSingleInstance()
-	
-	t.Log("释放空锁测试通过")
+func TestSingletonLock_DuplicateLockFails(t *testing.T) {
+	lockPath := filepath.Join(os.TempDir(), "nex-gateway-test-dup.lock")
+	defer os.Remove(lockPath)
+
+	lock1 := NewSingletonLock(lockPath)
+	if err := lock1.Lock(); err != nil {
+		t.Fatalf("首次加锁应成功: %v", err)
+	}
+	defer lock1.Unlock()
+
+	lock2 := NewSingletonLock(lockPath)
+	err := lock2.Lock()
+	if err == nil {
+		lock2.Unlock()
+		t.Fatal("重复加锁应失败，但返回 nil")
+	}
+}
+
+func TestSingletonLock_UnlockThenRelock(t *testing.T) {
+	lockPath := filepath.Join(os.TempDir(), "nex-gateway-test-relock.lock")
+	defer os.Remove(lockPath)
+
+	lock1 := NewSingletonLock(lockPath)
+	if err := lock1.Lock(); err != nil {
+		t.Fatalf("首次加锁应成功: %v", err)
+	}
+	lock1.Unlock()
+
+	lock2 := NewSingletonLock(lockPath)
+	if err := lock2.Lock(); err != nil {
+		t.Fatalf("释放后重新加锁应成功: %v", err)
+	}
+	lock2.Unlock()
+}
+
+func TestSingletonLock_UnlockWithoutLock(t *testing.T) {
+	lock := NewSingletonLock(filepath.Join(os.TempDir(), "nex-gateway-test-nil.lock"))
+	lock.Unlock()
 }