core: frontend: Add option to set custom subnet mask in network creation UI

### Current behaviour

Describe what you see:
Currently, when configuring a network in BlueOS via the web interface, users can specify an IP address—but there’s no option to explicitly set the subnet mask (e.g. /24 vs /16).

Why that is bad:
Without the ability to adjust the subnet mask, integrating BlueOS into larger or segmented networks becomes difficult

### Expected or desired behaviour

Explain what you expect:
Add a field to the network configuration interface that allows users to explicitly specify the subnet mask—either in CIDR notation (e.g., /24) or in full dotted‑decimal format (e.g., 255.255.255.0).

Why:
This option empowers users to configure networks according to project or organizational needs.

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start by locating the BlueOS web interface's network configuration screen and trace how its IP address is collected and submitted. Decide whether the new value should use CIDR or dotted-decimal notation based on the existing network API, then verify that creating a network preserves the explicitly chosen subnet mask.

Written by the indexing model from the issue text.