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/handler/vpn.go

@@ -406,3 +406,26 @@ func DeleteVpnReservation(c *gin.Context) {
 	}
 	c.JSON(http.StatusOK, gin.H{"message": "删除成功"})
 }
+
+func KickUserClient(c *gin.Context) {
+	idStr := c.Param("id")
+	id, err := strconv.ParseUint(idStr, 10, 64)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "参数错误"})
+		return
+	}
+
+	var user model.User
+	if err := db.DB.First(&user, id).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "用户不存在"})
+		return
+	}
+
+	if vpn.VPN == nil || !vpn.VPN.Running() {
+		c.JSON(http.StatusOK, gin.H{"message": "VPN 服务未运行", "kicked": 0})
+		return
+	}
+
+	n := vpn.VPN.KickUser(uint(id))
+	c.JSON(http.StatusOK, gin.H{"message": "已断开用户连接", "kicked": n})
+}