fix(admin): allow owner and admin account roles in user creation and role management

Four places were hardcoded to engineer|viewer only:
- AccountRoleUpdate schema (user.py) — blocked PUT /admin/users/{id}/account-role at the API level
- AdminUserCreate schema (admin.py) — blocked creating users with owner/admin role
- AccountDetailPage role dropdowns (create form + inline member role changer)
- AccountsPage create user role dropdown

Now all four accept the full set: owner, admin, engineer, viewer.

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

frontend/src/types/admin.ts

@@ -266,7 +266,7 @@ export interface AdminUserCreate {
   name: string
   account_mode: 'existing' | 'personal'
   account_display_code?: string
-  account_role?: 'engineer' | 'viewer'
+  account_role?: 'owner' | 'admin' | 'engineer' | 'viewer'
   send_email: boolean
 }