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/ui/elevation_other.go

@@ -0,0 +1,28 @@
+//go:build !darwin && !windows
+
+package ui
+
+import (
+	"fmt"
+	"os/exec"
+
+	"lmvpn/internal/log"
+)
+
+const daemonBinaryName = "lmvpnd"
+
+// launchElevated launches the daemon-launch subcommand with admin
+// privileges. On Linux, pkexec is used (common on desktop Linux).
+func launchElevated(exe, daemonBin, home string, uid, gid int) error {
+	cmd := exec.Command("pkexec", exe, "daemon-launch",
+		"--user-home", home,
+		"--uid", fmt.Sprintf("%d", uid),
+		"--gid", fmt.Sprintf("%d", gid),
+		"--daemon-bin", daemonBin)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("launch daemon: %w (output: %s)", err, string(out))
+	}
+	log.L().Info("daemon launched via pkexec",
+		"uid", uid, "gid", gid, "home", home, "daemon_bin", daemonBin)
+	return nil
+}