fix: align frontend API types and error handling with backend contracts

- updateRole return type matches backend's {id, role} instead of full AdminUser
- fetchFile error priority aligned with central client (message before error)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/admin.ts

@@ -46,6 +46,6 @@ export const adminUsersApi = {
     api.get<AdminUser[]>('/admin/users'),
 
   /** PUT /admin/users/:id/role -- promote or demote a user (admin/user). */
-  updateRole: (id: string, data: UpdateUserRoleRequest): Promise<AdminUser> =>
+  updateRole: (id: string, data: UpdateUserRoleRequest): Promise<{ id: string; role: string }> =>
-    api.put<AdminUser>(`/admin/users/${id}/role`, data),
+    api.put<{ id: string; role: string }>(`/admin/users/${id}/role`, data),
 };

frontend/src/api/syntheses.ts

@@ -50,7 +50,7 @@ export async function fetchFile(path: string): Promise<Response> {
     const errorBody = await response.json().catch(() => ({ error: 'Unknown error' }));
     throw {
       status: response.status,
-      message: errorBody.error || errorBody.message || `HTTP ${response.status}`,
+      message: errorBody.message || errorBody.error || `HTTP ${response.status}`,
     };
   }