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/handoff/multi-tenancy.md

@@ -103,7 +103,7 @@ conflicts.
 
 1. Every requirement MT-1 to MT-23 is implemented, or deliberately deferred
    with a line in the spec saying so. The branding and quota warnings for
-   ihasmail can wait for ihasmail-inbuxa.
+   ihasmail can wait for inbuxa-webmail.
 2. Acceptance tests 1 to 14 pass as integration tests
    (`tests/src/system/tenant.rs`), with the `pending-rebuild` gate removed
    from the tenant call.