feat: implement BackButton component for consistent navigation

- Added BackButton component to standardize back navigation across pages.
- Integrated BackButton into various pages, replacing custom back buttons for consistency.
- Updated PageHeader to accept backTo prop for displaying BackButton when navigating from subpages.
- Created useGoBack hook to handle navigation logic, determining whether to go back in history or redirect to a fallback page.
- Added tests for BackButton and its integration with PageHeader to ensure expected behavior.

assets/admin/pages/UserDetailPage.tsx

@@ -16,6 +16,7 @@ import { api } from '../lib/api';
 import type { ApiResponse } from '../lib/api';
 import { formatDate, formatDateTime } from '../lib/utils';
 import Modal from '../components/ui/Modal';
+import BackButton from '../components/ui/BackButton';
 import ConfirmDialog from '../components/ui/ConfirmDialog';
 
 // ── Types ─────────────────────────────────────────────────────────────────
@@ -333,6 +334,8 @@ export default function UserDetailPage() {
   return (
     <div className="animate-slide-up space-y-5">
 
+      <BackButton fallback="/admin/users" />
+
       {/* ── Breadcrumb / Back ─────────────────────────────────── */}
       <div className="flex items-center gap-2 text-sm text-[var(--text-2)]">
         <button