Allow Custom DHCP Range for Isolated Guest Networks

Currently, in isolated guest networks (with Source NAT), DHCP ranges are automatically derived from the network CIDR and cannot be explicitly defined.

Currently, users rely on CIDR adjustments by editing the network after it is created. It would be beneficial to allow administrators to directly specify the desired DHCP range during network creation for better flexibility and usability.

This feature would allow administrators to explicitly specify a DHCP range (e.g., 192.168.100.10 - 192.168.100.200) at the time of isolated guest network creation.

首先追蹤隔離來賓網路的建立流程，以及 DHCP 範圍從網路 CIDR 衍生的位置。定義在建立期間如何接受明確提供的範圍，例如 192.168.100.10–192.168.100.200，然後驗證所要求的範圍會被保留並用於 DHCP 設定。

由索引模型根據 Issue 內容生成。