Refactor frontend API calls into hooks and complete task states

frontend/src/providers/TimezoneProvider.tsx

@@ -9,15 +9,8 @@
  * always receive a safe fallback.
  */
 
-import {
-  createContext,
-  useCallback,
-  useContext,
-  useEffect,
-  useMemo,
-  useState,
-} from "react";
-import { fetchTimezone } from "../api/setup";
+import { createContext, useContext, useMemo } from "react";
+import { useTimezoneData } from "../hooks/useTimezoneData";
 
 // ---------------------------------------------------------------------------
 // Context definition
@@ -52,19 +45,7 @@ export interface TimezoneProviderProps {
 export function TimezoneProvider({
   children,
 }: TimezoneProviderProps): React.JSX.Element {
-  const [timezone, setTimezone] = useState<string>("UTC");
-
-  const load = useCallback((): void => {
-    fetchTimezone()
-      .then((resp) => { setTimezone(resp.timezone); })
-      .catch(() => {
-        // Silently fall back to UTC; the backend may not be reachable yet.
-      });
-  }, []);
-
-  useEffect(() => {
-    load();
-  }, [load]);
+  const { timezone } = useTimezoneData();
 
   const value = useMemo<TimezoneContextValue>(() => ({ timezone }), [timezone]);