Standardise frontend hook fetch error handling and mark Task 12 done

frontend/src/hooks/useTimezoneData.ts

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from "react";
 import { fetchTimezone } from "../api/setup";
+import { handleFetchError } from "../utils/fetchError";
 
 export interface UseTimezoneDataResult {
   timezone: string;
@@ -21,7 +22,7 @@ export function useTimezoneData(): UseTimezoneDataResult {
       const resp = await fetchTimezone();
       setTimezone(resp.timezone);
     } catch (err: unknown) {
-      setError(err instanceof Error ? err.message : "Failed to fetch timezone");
+      handleFetchError(err, setError, "Failed to fetch timezone");
       setTimezone("UTC");
     } finally {
       setLoading(false);