fix: TUN 子网路由缺失导致客户端无下载流量

- service.go: 移除无意义的 peerIP = serverIP（对端指向自己），改为 nil
- service.go: Configure 之后新增 AddSubnetRoute 调用，确保内核有到 VPN 子网的路由
- tun_linux.go: Configure 增加 peerIP != nil 守卫，nil 时直接走无 peer 分支
- tun_darwin.go: 同上 peerIP != nil 守卫

internal/vpn/tun_darwin.go

@@ -20,13 +20,10 @@ func (t *TUNInterface) Configure(localIP net.IP, prefix int, peerIP net.IP) erro
 	}
 	localCidr := fmt.Sprintf("%s/%d", localIP.String(), prefix)
 	inetType := inetFamily(localIP)
-	var err error
-	if t.Iface.IsTUN() && inetType == "inet" {
-		err = execCmd("ifconfig", t.Name(), inetType, localCidr, peerIP.String(), "up")
-	} else {
-		err = execCmd("ifconfig", t.Name(), inetType, localCidr, "up")
+	if t.Iface.IsTUN() && inetType == "inet" && peerIP != nil {
+		return execCmd("ifconfig", t.Name(), inetType, localCidr, peerIP.String(), "up")
 	}
-	return err
+	return execCmd("ifconfig", t.Name(), inetType, localCidr, "up")
 }
 
 func (t *TUNInterface) AddSubnetRoute(subnet *net.IPNet) error {