Move auth and timezone hooks into dedicated hook files

frontend/src/providers/TimezoneProvider.tsx

@@ -9,19 +9,19 @@
  * always receive a safe fallback.
  */
 
-import { createContext, useContext, useMemo } from "react";
+import { createContext, useMemo } from "react";
 import { useTimezoneData } from "../hooks/useTimezoneData";
 
 // ---------------------------------------------------------------------------
 // Context definition
 // ---------------------------------------------------------------------------
 
-interface TimezoneContextValue {
+export interface TimezoneContextValue {
   /** IANA timezone string, e.g. ``"Europe/Berlin"`` or ``"UTC"``. */
   timezone: string;
 }
 
-const TimezoneContext = createContext<TimezoneContextValue>({ timezone: "UTC" });
+export const TimezoneContext = createContext<TimezoneContextValue | undefined>(undefined);
 
 // ---------------------------------------------------------------------------
 // Provider
@@ -53,24 +53,3 @@ export function TimezoneProvider({
     <TimezoneContext.Provider value={value}>{children}</TimezoneContext.Provider>
   );
 }
-
-// ---------------------------------------------------------------------------
-// Hook
-// ---------------------------------------------------------------------------
-
-/**
- * Return the IANA timezone string configured during setup.
- *
- * Must be used inside a {@link TimezoneProvider}.
- *
- * @returns The configured timezone, e.g. ``"Europe/Berlin"``.
- *
- * @example
- * ```tsx
- * const { timezone } = useTimezone();
- * const label = formatDate(item.created_at, timezone);
- * ```
- */
-export function useTimezone(): string {
-  return useContext(TimezoneContext).timezone;
-}