chore(dashboard): remove superseded /admins page

/admin/users is a strict superset of the pre-3.5 /admin/admins
page (adds role chip, email column, search, role-aware affordance
hiding, and the password-reveal flow), so the old page would only
split traffic and confuse muscle memory.

Also drops the AdminUserRecord type alias kept in place to ease
the transition.

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

dashboard/src/lib/api.ts

@@ -274,9 +274,6 @@ export interface AdminDto {
 	last_login_at: string | null;
 }
 
-/** @deprecated use AdminDto. Kept until the /admins route is retired. */
-export type AdminUserRecord = AdminDto;
-
 export interface CreateAdminInput {
 	username: string;
 	password: string;