feat: VPN 管理后台新增系统环境检测面板

- 新增 /api/admin/vpn/diag 端点，检测 TUN 创建能力、root、CAP_NET_ADMIN、ip_forward、MASQUERADE
- Linux 下解析 /proc/self/status CapEff 位判断 CAP_NET_ADMIN，读 /proc/sys/net/ipv4/ip_forward，执行 iptables 检查 MASQUERADE
- 非 Linux 平台显示"仅 Linux 适用"
- 前端 VpnView 新增系统环境检测卡片，每项以勾叉标识并附修复提示

internal/router/router.go

@@ -38,6 +38,7 @@ func Setup(r *gin.Engine) {
 		admin.GET("/vpn/settings", handler.GetVpnSettings)
 		admin.PUT("/vpn/settings", handler.UpdateVpnSettings)
 		admin.GET("/vpn/status", handler.GetVpnStatus)
+		admin.GET("/vpn/diag", handler.GetVpnDiag)
 		admin.GET("/vpn/reservations", handler.ListVpnReservations)
 		admin.POST("/vpn/reservations", handler.CreateVpnReservation)
 		admin.DELETE("/vpn/reservations/:id", handler.DeleteVpnReservation)