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_windows.go

@@ -0,0 +1,44 @@
+//go:build windows
+
+package ui
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+
+	"lmvpn/internal/log"
+)
+
+const daemonBinaryName = "lmvpnd.exe"
+
+// launchElevated launches the daemon-launch subcommand with UAC
+// elevation via ShellExecute "runas" on Windows.
+func launchElevated(exe, daemonBin, home string, uid, gid int) error {
+	args := fmt.Sprintf("daemon-launch --user-home %s --uid %d --gid %d --daemon-bin %s",
+		home, uid, gid, daemonBin)
+
+	shell32 := syscall.NewLazyDLL("shell32.dll")
+	proc := shell32.NewProc("ShellExecuteW")
+
+	verb, _ := syscall.UTF16PtrFromString("runas")
+	file, _ := syscall.UTF16PtrFromString(exe)
+	params, _ := syscall.UTF16PtrFromString(args)
+	dir, _ := syscall.UTF16PtrFromString("")
+
+	ret, _, err := proc.Call(
+		0,
+		uintptr(unsafe.Pointer(verb)),
+		uintptr(unsafe.Pointer(file)),
+		uintptr(unsafe.Pointer(params)),
+		uintptr(unsafe.Pointer(dir)),
+		1, // SW_SHOWNORMAL
+	)
+
+	if ret <= 32 {
+		return fmt.Errorf("UAC elevation failed (code %d): %w", ret, err)
+	}
+	log.L().Info("daemon launched via UAC",
+		"uid", uid, "gid", gid, "home", home, "daemon_bin", daemonBin)
+	return nil
+}