fix(frontend): prevent memory leak in useIdempotentAction.ts (#1221)

[Mercy017]

Root cause

useIdempotentAction's useEffect cleanup only set isMountedRef.current = false. The inFlightActions ref (Map<string, Promise<unknown>>) was never cleared on unmount.

Any Promise stored in that Map — together with the closures it captured (state setters, isProcessingRef, lastExecutionTime, etc.) — was retained in memory until the Promise settled. On high-churn routes where the hook's component mounts and unmounts frequently, these retained closures accumulate and prevent GC of the entire component subtree.

Fix (src/hooks/useIdempotentAction.ts)

Added inFlightActions.current.clear() to the useEffect cleanup so in-flight Promise references are released the moment the component unmounts, allowing the GC to reclaim them.

  return () => {
    isMountedRef.current = false;
+   inFlightActions.current.clear();
  };

Regression test (src/hooks/__tests__/useIdempotentAction.test.ts)

New test "regression: clears inFlightActions on unmount …":

1. Starts an action, leaving a Promise in inFlightActions.
2. Unmounts the component while the Promise is in flight.
3. Resolves the Promise after unmount — verifies no stale-state update throws.
4. Confirms a fresh hook instance executes normally, proving no lingering state.

Closes #1221
Closes #1220

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