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/README.md

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+# React Hook Fetch Cancellation
+
+This folder follows a shared convention for network fetch cancellation in React hooks.
+
+## Patterns
+
+### 1. Hooks with manual refresh
+
+Hooks that expose a `refresh()` callback must use a long-lived `AbortController` stored in a ref:
+
+- `const abortRef = useRef<AbortController | null>(null);
+- Call `abortRef.current?.abort()` before starting a new request.
+- Create a fresh controller before every `refresh()` invocation.
+- Pass `controller.signal` to the API function.
+- In the cleanup effect, abort the controller when the hook unmounts.
+- After each `await`, check `signal.aborted` before updating state.
+
+This prevents stale responses from overwriting newer results and avoids React state updates after unmount.
+
+### 2. One-shot mount-only requests
+
+Hooks that only fetch once inside `useEffect` and do not expose a manual refresh may use a local controller:
+
+- Create `const controller = new AbortController();` inside the effect.
+- Pass `controller.signal` to the request.
+- Abort it in the effect cleanup.
+- This is the simplest correct pattern for single-fetch hooks.
+
+### 3. Do not use boolean cancelled flags for network requests
+
+A boolean `cancelled` flag is not sufficient because it does not stop the underlying fetch. Abort signals are the correct cancellation mechanism for fetch-based hooks.