feat: 新增管理员踢下线 VPN 客户端功能

- vpn/service.go: ClientInfo 增加 user_id 字段，新增 KickUser 方法
  用读锁收集目标连接后逐个关闭 WebSocket，返回断开数量
- vpn/tunnel.go: info() 填充 UserID
- handler/vpn.go: 新增 KickUserClient handler，校验用户存在后调用 KickUser
- router.go: 注册 DELETE /api/admin/vpn/clients/:id
- VpnView.vue: 在线客户端表格新增操作列与踢下线按钮，handleKick 确认后调用 API 并刷新
- zh.ts/en.ts: 新增 kick/confirmKick/kickFailed 国际化 key

internal/vpn/service.go

@@ -374,10 +374,26 @@ func (s *VpnService) ClientList() []ClientInfo {
 }
 
 type ClientInfo struct {
+	UserID      uint   `json:"user_id"`
 	Username    string `json:"username"`
 	IP          string `json:"ip"`
 	IP6         string `json:"ip6,omitempty"`
 	ConnectedAt string `json:"connected_at"`
 }
 
+func (s *VpnService) KickUser(userID uint) int {
+	s.mu.RLock()
+	var toClose []*tunnelConn
+	for c := range s.clients {
+		if c.user.ID == userID {
+			toClose = append(toClose, c)
+		}
+	}
+	s.mu.RUnlock()
+	for _, c := range toClose {
+		c.close()
+	}
+	return len(toClose)
+}
+
 var VPN *VpnService

internal/vpn/tunnel.go

@@ -79,6 +79,7 @@ func (c *tunnelConn) close() {
 
 func (c *tunnelConn) info() ClientInfo {
 	ci := ClientInfo{
+		UserID:      c.user.ID,
 		Username:    c.user.Username,
 		IP:          c.assignedIP.String(),
 		ConnectedAt: c.connectedAt.Format("2006-01-02 15:04:05"),