feat: enhance testing and admin page functionality

- Mocked `useRouter` from `next/navigation` in tests to improve routing behavior during testing.
- Updated admin page tests to reflect changes in title display and removed unnecessary back link check.
- Refactored admin page header to improve accessibility and layout, displaying month and year more clearly.
- Removed unused imports and components to streamline code and enhance maintainability.

webapp-next/src/components/calendar/CalendarHeader.tsx

@@ -5,7 +5,6 @@
 
 "use client";
 
-import type { ReactNode } from "react";
 import { Button } from "@/components/ui/button";
 import { useTranslation } from "@/i18n/use-translation";
 import { cn } from "@/lib/utils";
@@ -21,8 +20,6 @@ export interface CalendarHeaderProps {
   disabled?: boolean;
   onPrevMonth: () => void;
   onNextMonth: () => void;
-  /** Optional content shown above the nav row (e.g. Admin link). */
-  trailingContent?: ReactNode;
   className?: string;
 }
 
@@ -31,7 +28,6 @@ export function CalendarHeader({
   disabled = false,
   onPrevMonth,
   onNextMonth,
-  trailingContent,
   className,
 }: CalendarHeaderProps) {
   const { t, monthName, weekdayLabels } = useTranslation();
@@ -41,9 +37,6 @@ export function CalendarHeader({
 
   return (
     <header className={cn("flex flex-col", className)}>
-      {trailingContent != null && (
-        <div className="flex justify-end mb-1 min-h-[1.5rem]">{trailingContent}</div>
-      )}
       <div className="flex items-center justify-between mb-3">
         <Button
           type="button"