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/hooks/useSetup.ts

@@ -4,7 +4,7 @@
  * Exposes the current setup completion status and a submission handler.
  */
 
-import { useCallback, useEffect, useState } from "react";
+import { useCallback, useEffect, useRef, useState } from "react";
 import { ApiError } from "../api/client";
 import { handleFetchError } from "../utils/fetchError";
 import { getSetupStatus, submitSetup } from "../api/setup";
@@ -36,24 +36,42 @@ export function useSetup(): UseSetupResult {
   const [error, setError] = useState<string | null>(null);
   const [submitting, setSubmitting] = useState(false);
   const [submitError, setSubmitError] = useState<string | null>(null);
+  const abortRef = useRef<AbortController | null>(null);
 
   const refresh = useCallback(async (): Promise<void> => {
+    abortRef.current?.abort();
+    const controller = new AbortController();
+    abortRef.current = controller;
+    const { signal } = controller;
+
     setLoading(true);
     setError(null);
 
     try {
-      const resp = await getSetupStatus();
+      const resp = await getSetupStatus(signal);
+      if (signal.aborted) {
+        return;
+      }
       setStatus(resp);
     } catch (err: unknown) {
+      if (signal.aborted) {
+        return;
+      }
       const fallback = "Failed to fetch setup status";
       handleFetchError(err, setError, fallback);
     } finally {
-      setLoading(false);
+      if (!signal.aborted) {
+        setLoading(false);
+      }
     }
   }, []);
 
   useEffect(() => {
     void refresh();
+
+    return (): void => {
+      abortRef.current?.abort();
+    };
   }, [refresh]);
 
   const submit = useCallback(async (payload: SetupRequest): Promise<void> => {