feat(admin): allow setting owner when creating an account

Adds optional owner_email field to the Create Account modal. Superadmin
can specify an existing user's email to assign as account owner at
creation time. Backend 404s with a clear message if the email is unknown.
Error detail now surfaces to the toast instead of a generic message.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/admin.ts

@@ -114,6 +114,7 @@ export interface AdminAccountDetailResponse extends AdminAccountListItem {
 export interface AdminAccountCreate {
   name: string
   plan: 'free' | 'pro' | 'team'
+  owner_email?: string
 }
 
 export interface AdminAccountUpdate {