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/app/(portal)/admin/layout.tsx

@@ -0,0 +1,41 @@
+'use client';
+
+import Link from 'next/link';
+import { useTranslations } from 'next-intl';
+import { AdminSidebar } from '@/components/admin/admin-sidebar';
+
+export default function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  const t = useTranslations('settings');
+
+  return (
+    <div className="flex h-full">
+      <AdminSidebar />
+      <div className="flex-1 overflow-y-auto p-6">
+        <Link
+          href="/"
+          className="mb-4 inline-flex items-center gap-1 text-sm text-muted-foreground transition-colors hover:text-foreground"
+        >
+          <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"
+          >
+            <polyline points="15 18 9 12 15 6" />
+          </svg>
+          {t('backToDashboard')}
+        </Link>
+        {children}
+      </div>
+    </div>
+  );
+}

apps/web/src/app/(portal)/admin/users/page.tsx

@@ -253,7 +253,8 @@ export default function AdminUsersPage() {
                       </button>
                       <button
                         onClick={() => setDeleteConfirm(user.id)}
-                        className="rounded px-2 py-1 text-xs text-destructive hover:bg-destructive/10 transition-colors"
+                        disabled={user.id === currentUser?.id}
+                        className="rounded px-2 py-1 text-xs text-destructive hover:bg-destructive/10 transition-colors disabled:opacity-30 disabled:cursor-not-allowed disabled:pointer-events-none"
                       >
                         {tCommon('delete')}
                       </button>