feat(installer): Prompt for the Device Agent port in interactive mode

[ppawlowski]

Description

This pull request adds a port question to the installer's interactive mode, so a Remote Instance integrator can choose which TCP port the Device Agent binds to without having to know the --port flag up front. The installer asks for the
port straight after the permission check and re-asks until the answer is a free port in the 1025-65535 range. Passing --port still works unchanged and skips the question, reporting only which port will be used.

--port is now a nullable flag value, so an explicit --port 1880 stays distinguishable from an omitted flag, and range validation moved into the flag type - also lets pflag reject a bad value itself and corrects the help
text, previously advertised as 1-65535 while the guard rejected anything below 1025.

When the chosen port is not the default, the installation directory offered by the next question carries it (/opt/flowfuse-device-1881), so agents on different ports are not proposed the same location. It remains a suggestion and any absolute path can be typed instead. Scripted installations driven by --otc stay non-interactive throughout and keep the plain default directory.

Additionally, this pull request:

• fixes free port check method - now it checks all available network interfaces instead of loopback only
• adjusts port validation approach
• reorganizes the utils package

Related Issue(s)

Closes #730

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[Steve-Mcl]

Cool! Local testing good on windows,

[Steve-Mcl]

one v minor niggle - the line "The FlowFuse Device Agent will use port 1888." is output after user choice, but there is no equivalent "Install Dir: C:\opt\flowfuse-device-1888" output after the directory choice!