HaikuDepot: Refactor usage conditions windows.

* Set only a minimum width and then resize to preferred on window.
* Set a minimum height for the text view itself.
* Use the new TextView controls instead of the old BTextView.
* Adjust other size constraints as needed.

Fixes #17998.

HaikuDepot: Check User Auth on Start

The user might have changed their authentication details
on the server and the client won't detect this until
they go to do something. Instead, if possible, check
this as the client starts. Also check that the user has
agreed to the current user usage conditions.

As a side-effect this generalizes the logic for process
coordination in the main window and also fixes some bugs
in the main window's progress display as the application
starts.

Relates to #15209

Change-Id: I4c9620648819ecd14fb095e4cb2c66fe7b2a0920
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2467
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>