Update form and validation logic in connections

Rework the form handling and validation steps of the add and edit connection dialog.

This could use `react-hook-form` as we are currently, or transition to the better `tanstack-form` package.

Start by locating the add and edit connection dialogs and their current react-hook-form usage. Compare the existing form handling and validation with the proposed tanstack-form option; the work is done when both dialogs use the agreed approach and their validation behavior is covered.

Written by the indexing model from the issue text.