Make network configuration a mandatory menu selection

[Softer]

• This fix issue:

PR Description:

Users frequently forget to configure network during installation, resulting in no internet after reboot. And I think there are not many setups without a network :)

This PR makes the network selection item mandatory (same pattern as Disk configuration). Users must explicitly choose one of the options before Install becomes available.

A new "No network" option is added for users who intentionally want no network on the target system.

The network options list follows the enum order instead of alphabetical sorting, with "No network" at the bottom.

A recommendation hint is shown in the menu header.

Tests and Checks

• Network not selected -> Install shows "Missing configurations: Network configuration"
• Select any network option -> Install unblocked
• Select "No network" -> Install unblocked, no network packages installed
• Old config with network_config via --config -> works as before
• Old config without network_config -> mandatory prompt in interactive mode

[svartkanin]

I'm not sure this is a good argument, users should not be forced into selections that are not required for an installation.
Right now only the required settings for a working system are required and this deviates from that pattern.

[Softer]

I agree with the principle of minimalism, but I think that in 2026, the network, in most cases, meets the minimum requirements.

[svartkanin]

Considering there is a fair amount of people installing it on VMs and such it may still be somewhat impactfull