Standardise AbortController cancellation in setup and server health hooks

Add abortable API signals for setup status and server health/log fetches, document hook cancellation patterns, and cover stale refresh cancellation with tests.

frontend/src/api/setup.ts

@@ -18,8 +18,8 @@ import type {
  *
  * @returns Setup status response with a `completed` boolean.
  */
-export async function getSetupStatus(): Promise<SetupStatusResponse> {
-  return api.get<SetupStatusResponse>(ENDPOINTS.setup);
+export async function getSetupStatus(signal?: AbortSignal): Promise<SetupStatusResponse> {
+  return api.get<SetupStatusResponse>(ENDPOINTS.setup, signal);
 }
 
 /**