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/app/admin/page.tsx

@@ -6,10 +6,8 @@
 
 "use client";
 
-import Link from "next/link";
 import { useAdminPage, AdminDutyList, ReassignSheet } from "@/components/admin";
 import { useTranslation } from "@/i18n/use-translation";
-import { Button } from "@/components/ui/button";
 import { AccessDeniedScreen } from "@/components/states/AccessDeniedScreen";
 import { LoadingState } from "@/components/states/LoadingState";
 import { ErrorState } from "@/components/states/ErrorState";
@@ -47,24 +45,28 @@ export default function AdminPage() {
           <p className="text-muted-foreground">
             {admin.adminAccessDeniedDetail ?? t("admin.access_denied")}
           </p>
-          <Button asChild variant="outline">
-            <Link href="/">{t("admin.back_to_calendar")}</Link>
-          </Button>
         </div>
       </div>
     );
   }
 
+  const month = admin.currentMonth.getMonth();
+  const year = admin.currentMonth.getFullYear();
+
   return (
     <div className={PAGE_WRAPPER_CLASS}>
-      <header className="sticky top-0 z-10 flex items-center justify-between border-b bg-[var(--header-bg)] py-3">
-        <h1 className="text-lg font-semibold">
-          {t("admin.title")} — {monthName(admin.currentMonth.getMonth())}{" "}
-          {admin.currentMonth.getFullYear()}
+      <header className="sticky top-0 z-10 flex flex-col items-center border-b bg-[var(--header-bg)] py-3">
+        <h1
+          className="m-0 flex flex-col items-center justify-center gap-0 leading-none"
+          aria-label={`${t("admin.title")}, ${monthName(month)} ${year}`}
+        >
+          <span className="text-xs font-normal leading-none text-muted">
+            {year}
+          </span>
+          <span className="text-[1.1rem] font-semibold leading-tight sm:text-[1.25rem]">
+            {monthName(month)}
+          </span>
         </h1>
-        <Button asChild variant="ghost" size="sm">
-          <Link href="/">{t("admin.back_to_calendar")}</Link>
-        </Button>
       </header>
 
       {admin.successMessage && (