fix: validate each setup answer at the prompt that asked for it

Five defects from the user's hand test of the interactive setup, four of
them the same mistake: an answer was validated somewhere other than where
it was given, so a typo cost the whole run.

A bad CIDR was only rejected after the NEXT question had been asked and
answered, and both answers were then discarded. It is validated at its own
prompt now and re-asks, so a mistake costs one retry.

A provider name typed at the first prompt was treated as a CIDR, so
"Gmail" produced "does not appear to be an IPv4 or IPv6 network", which
names nothing the user can act on. The prompt takes either answer now and
tries the provider table first, because the user does not know, and should
not have to know, which of the two questions they are answering.

Picking a hop number out of range raised a bare IndexError, and a
non-numeric pick leaked int()'s own message about base 10. Both re-ask
naming the valid range.

The first question said what to type without saying what it was for. It
now states that the boundary decides which address gets reported, which is
the fact that makes a wrong answer matter.

Ctrl+C and EOF are caught in main() and report that nothing was written.
Setup writes nothing until every answer is in hand, so abandoning it
midway leaves no half-written config, and that was already true; it just
said so with a traceback.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KphFXTc2QajxXsHWyvGJ4R

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