Add-host: default repo username to hostname; always show htpasswd snippet

The pending page suppressed the htpasswd snippet when repo_username
was blank — but with --private-repos the username is required for
auth, and operators routinely leave the field blank assuming the
system will pick something sensible.

* handleUIAddHostPost defaults repo_username to the typed hostname
  when blank. Matches what --private-repos expects (URL path
  segment == username).
* pending_host.html: snippet now renders whenever a password is
  present (always true after the generate-on-blank logic landed
  earlier).
* Form help-text updated to describe the default explicitly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/pages/pending_host.html

@@ -27,7 +27,7 @@
     the host detail.
   </p>
 
-  {{if and $page.RepoUsername $page.RepoPassword}}
+  {{if $page.RepoPassword}}
   <div class="snippet mt-6 panel" style="border-color: color-mix(in oklch, var(--warn), transparent 60%);">
     <div class="snippet-head" style="background: color-mix(in oklch, var(--warn), transparent 92%);">
       <span>