Call the webmail inbuxa-webmail in docs and comments

The webmail repository was renamed from ihasmail-inbuxa to inbuxa-webmail
on 2026-10-05. The OAuth client id stays ihasmail-inbuxa: that is what the
server registers, so the backticked and quoted ids are unchanged.

docs/spec/cutover.md

@@ -584,7 +584,7 @@ Three consequences:
   name whose DNS points at the mail addresses.
 - Whether the front ends need anything at cutover, or follow separately
   (SPEC.md §5). The rehearsal does not start them.
-- Whether ihasmail-inbuxa and INBUXA Admin behave under real use, rather
+- Whether inbuxa-webmail and INBUXA Admin behave under real use, rather
   than at first sign-in. Both were verified as far as signing in and, for the
   webmail, mail flowing.
 - The checks only users can make: the second account, the mailbox comparison