panel: hide domain pick for global administrators on user form

Global administrators manage every domain; the assignment checkboxes are
now hidden when that role is selected.

Co-authored-by: Cursor <cursoragent@cursor.com>

CHANGELOG.md

@@ -21,6 +21,8 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Changed
 
+- panel: the user create/edit form hides **Assigned domains** when the role is
+  global administrator, since that role manages every domain anyway.
 - panel: **Settings** shows panel credentials and DMARC aggregate reports side
   by side for global administrators (the same `.split` layout as a delivery's
   message and history). Domain-scoped users keep the single narrow card.