fix(delete): review follow-ups — admin Deleted filter, guest gate, dialog spacing

- Admin /users gains a Deleted scope (between People and Robots); every other scope now
  hides tombstoned accounts (deleted_at filter in UserFilter).
- Admin delete-user is offered for any non-deleted account (drop the not-guest gate, so a
  stale is_guest account can still be tombstoned).
- Harden EmailService.ConfirmCode to ClearGuest — defence-in-depth so no confirmed-email
  path leaves is_guest set (the currently-live paths already do).
- Space the delete/change dialog's action row from its input field.
Integration tests: the Deleted filter scoping + ConfirmCode guest promotion.

backend/internal/adminconsole/templates/pages/users.gohtml

@@ -2,11 +2,12 @@
 <h1>Users</h1>
 {{with .Data}}
 <nav class="subnav">
-<a href="/_gm/users"{{if not .Robots}} class="active"{{end}}>People</a> ·
+<a href="/_gm/users"{{if and (not .Robots) (not .Deleted)}} class="active"{{end}}>People</a> ·
+<a href="/_gm/users?kind=deleted"{{if .Deleted}} class="active"{{end}}>Deleted</a> ·
 <a href="/_gm/users?kind=robots"{{if .Robots}} class="active"{{end}}>Robots</a>
 </nav>
 <form class="form" method="get" action="/_gm/users">
-{{if .Robots}}<input type="hidden" name="kind" value="robots">{{end}}
+{{if .Robots}}<input type="hidden" name="kind" value="robots">{{end}}{{if .Deleted}}<input type="hidden" name="kind" value="deleted">{{end}}
 <input name="name" value="{{.NameMask}}" placeholder="display name mask (* ?)">
 <input name="ext" value="{{.ExternalIDMask}}" placeholder="external id mask (* ?)">
 <input name="email" value="{{.EmailExact}}" placeholder="email (exact)" type="search">