feat: 支持 Windows x86_64 平台编译

- 新增 WinTun TUN 设备实现(tun_windows.go),无需安装驱动,
  wintun.dll 嵌入 exe 运行时自动释放
- 新增 Windows 路由管理(route_windows.go),使用 route/netsh 命令
- 新增 Windows 路径(paths_windows.go),使用 %APPDATA%/%LOCALAPPDATA%
- 拆分 daemon 启动逻辑:launch_unix.go(Setsid) / launch_windows.go
  (CREATE_NEW_PROCESS_GROUP),公共代码提取到 launch_common.go
- 拆分提权逻辑:elevation_darwin.go(osascript) / elevation_windows.go
  (ShellExecute UAC) / elevation_other.go(pkexec)
- IPCSocketPath 按平台分散,Windows 用 %TEMP%\lmvpn.sock
- daemon 二进制名用编译时常量(daemonBinaryName),按平台选定 .exe 后缀
- Makefile 新增 build-windows 目标
- 重新标记 !darwin 文件为 !darwin && !windows

internal/daemon/launch_windows.go

@@ -0,0 +1,93 @@
+//go:build windows
+
+package daemon
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+
+	"lmvpn/internal/paths"
+)
+
+// Launch starts the daemon binary (lmvpnd.exe) as a detached process
+// and returns immediately. On Windows, detachment is achieved via
+// CREATE_NEW_PROCESS_GROUP | DETACHED_PROCESS creation flags instead
+// of the Unix Setsid call.
+//
+// The launcher itself is invoked by the GUI via ShellExecute "runas"
+// (UAC elevation). It:
+//  1. Computes the daemon log path using the user's home directory
+//  2. Opens the log file (append) and NUL
+//  3. Starts the lmvpnd.exe binary, redirecting stdio
+//  4. Returns immediately — the child continues running independently
+func Launch(userHome string, uid, gid int, daemonBin string) error {
+	paths.SetUserHome(userHome)
+	if err := paths.EnsureDirs(); err != nil {
+		fmt.Fprintf(os.Stderr, "ensure dirs: %v\n", err)
+	}
+
+	if daemonBin == "" {
+		var err error
+		daemonBin, err = resolveDaemonBinary()
+		if err != nil {
+			return err
+		}
+	}
+
+	absBin, err := filepath.Abs(daemonBin)
+	if err != nil {
+		return fmt.Errorf("resolve absolute path for %s: %w", daemonBin, err)
+	}
+	daemonBin = absBin
+
+	if _, err := os.Stat(daemonBin); err != nil {
+		return fmt.Errorf("daemon binary not found at %s: %w", daemonBin, err)
+	}
+
+	logPath := paths.DaemonLogFile()
+	logFile, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o644)
+	if err != nil {
+		return fmt.Errorf("open daemon log %s: %w", logPath, err)
+	}
+
+	devNull, err := os.Open(os.DevNull)
+	if err != nil {
+		logFile.Close()
+		return fmt.Errorf("open NUL: %w", err)
+	}
+
+	args := []string{
+		"--user-home", userHome,
+		"--uid", fmt.Sprintf("%d", uid),
+		"--gid", fmt.Sprintf("%d", gid),
+	}
+
+	fmt.Fprintf(logFile, "=== launcher: starting daemon %s %v ===\n", daemonBin, args)
+
+	// CREATE_NEW_PROCESS_GROUP (0x200) | DETACHED_PROCESS (0x8)
+	procAttr := &os.ProcAttr{
+		Dir: os.TempDir(),
+		Env: append(os.Environ(),
+			"LMVPN_DAEMON=1",
+		),
+		Files: []*os.File{devNull, logFile, logFile},
+		Sys: &syscall.SysProcAttr{
+			CreationFlags: 0x00000200 | 0x00000008,
+		},
+	}
+
+	pid, err := os.StartProcess(daemonBin, append([]string{daemonBin}, args...), procAttr)
+	logFile.Close()
+	devNull.Close()
+
+	if err != nil {
+		return fmt.Errorf("start daemon process: %w", err)
+	}
+
+	_ = pid.Release()
+
+	fmt.Fprintf(os.Stderr, "daemon launched (pid %d, bin %s)\n", pid.Pid, daemonBin)
+	return nil
+}