refactor(ui): restructure sidebar and admin navigation

- Sidebar: remove footer (user info, settings, locale switcher), remove admin section, make category headers static with per-category collapse toggle
- Header dropdown: replace inline admin links with single "Administrator" entry
- Admin section: dedicated layout with AdminSidebar (mirrors Settings pattern) at /admin/*
- Disable self-delete button in user management (backend already rejects with 403)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/components/layout/header.tsx

@@ -7,6 +7,7 @@ import { useSidebarStore } from '@/lib/stores/sidebar-store';
 import { useAuthStore } from '@/lib/stores/auth-store';
 import { fetchCurrentUser, logout } from '@/lib/auth-actions';
 import { ThemeToggle } from '@/components/theme-toggle';
+import { usePathname } from 'next/navigation';
 
 export function Header() {
   const t = useTranslations();
@@ -15,6 +16,9 @@ export function Header() {
   const { user, setUser } = useAuthStore();
   const [dropdownOpen, setDropdownOpen] = useState(false);
   const dropdownRef = useRef<HTMLDivElement>(null);
+  const pathname = usePathname();
+
+  const isAdmin = user?.role === 'ADMIN' || user?.role === 'SUPER_ADMIN';
 
   // Fetch current user on mount to populate auth store
   useEffect(() => {
@@ -154,7 +158,28 @@ export function Header() {
                 </Link>
               </div>
 
+              {/* Administration link */}
+              {isAdmin && (
+                <div className="px-2 py-1">
+                  <Link
+                    href="/admin/users"
+                    onClick={() => setDropdownOpen(false)}
+                    className={`flex w-full items-center gap-2 rounded-md px-2 py-1.5 text-sm transition-colors ${
+                      pathname.startsWith('/admin')
+                        ? 'bg-muted text-foreground font-medium'
+                        : 'text-foreground hover:bg-muted'
+                    }`}
+                  >
+                    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+                      <path d="M12 22s8-4 8-10V5l-8-3-8 3v7c0 6 8 10 8 10z" />
+                    </svg>
+                    {tHeader('admin.label')}
+                  </Link>
+                </div>
+              )}
+
               {/* Logout button */}
+              <div className="border-t border-border my-1" />
               <div className="px-2 py-1">
                 <button
                   onClick={() => {