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/pages/admin/AccountsPage.tsx

@@ -74,7 +74,7 @@ export function UsersPage() {
     name: '',
     account_mode: 'personal' as 'existing' | 'personal',
     account_display_code: '',
-    account_role: 'engineer' as 'engineer' | 'viewer',
+    account_role: 'engineer' as 'owner' | 'admin' | 'engineer' | 'viewer',
     send_email: true,
   })
   const [createLoading, setCreateLoading] = useState(false)
@@ -700,12 +700,14 @@ export function UsersPage() {
                 <label className="mb-1 block text-sm font-medium text-foreground">Account Role</label>
                 <select
                   value={createForm.account_role}
-                  onChange={(e) => setCreateForm((form) => ({ ...form, account_role: e.target.value as 'engineer' | 'viewer' }))}
+                  onChange={(e) => setCreateForm((form) => ({ ...form, account_role: e.target.value as 'owner' | 'admin' | 'engineer' | 'viewer' }))}
                   className={cn(
                     'w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
                     'focus:outline-hidden focus:border-primary focus:ring-2 focus:ring-primary/20'
                   )}
                 >
+                  <option value="owner">Owner</option>
+                  <option value="admin">Admin</option>
                   <option value="engineer">Engineer</option>
                   <option value="viewer">Viewer</option>
                 </select>