feat(admin): add user role dialog

Admins change a user's role and see the change log. The last admin
gets a clear error when deleting their own account.

README.md

@@ -119,7 +119,8 @@ To Be Played is split into three repositories:
   a repaired cover comes back as `imageUrl`.
 - German and English, dark mode, mobile layout, session-expired message instead of a silent logout.
 - **Admin** (PLATFORM_ADMIN only, `/admin/users`): search the users, block, unblock and delete them (the email is
-  typed to confirm a delete). "View as" in the menu switches the own session to User or Auditor and back; a badge
+  typed to confirm a delete), and set the role of another user in the "Roles" dialog, which also lists the role
+  changes. "View as" in the menu switches the own session to User or Auditor and back; a badge
   shows the lower role. Auth checks every right on the server. Registrations that wait show "Waits" with Accept
   and Deny (a "Waiting only" filter lists just them); the menu shows how many wait, loaded again on every page change
   and menu opening. A request whose address is not confirmed shows "Not confirmed" and only a Delete button. The login tells a waiting user that the admin has not decided yet.