Add AbortController cleanup to async frontend effects

frontend/src/hooks/useTimezoneData.ts

@@ -14,23 +14,31 @@ export function useTimezoneData(): UseTimezoneDataResult {
   const [loading, setLoading] = useState<boolean>(true);
   const [error, setError] = useState<string | null>(null);
 
-  const load = useCallback(async (): Promise<void> => {
+  const load = useCallback(async (signal?: AbortSignal): Promise<void> => {
     setLoading(true);
     setError(null);
 
     try {
-      const resp = await fetchTimezone();
+      const resp = await fetchTimezone(signal);
+      if (signal?.aborted) return;
       setTimezone(resp.timezone);
     } catch (err: unknown) {
+      if (signal?.aborted) return;
       handleFetchError(err, setError, "Failed to fetch timezone");
       setTimezone("UTC");
     } finally {
-      setLoading(false);
+      if (!signal?.aborted) {
+        setLoading(false);
+      }
     }
   }, []);
 
   useEffect(() => {
-    void load();
+    const controller = new AbortController();
+    void load(controller.signal);
+    return (): void => {
+      controller.abort();
+    };
   }, [load]);
 
   return {