feat(flutter/settings): admin card visible to admins

Settings gains an _AdminSection that renders an "Admin" tile (Shield
icon → /admin) only when the current profile has is_admin = true.
Non-admins see the section as SizedBox.shrink() so the const ListView
children stay const.

Provides a discoverable second entry to the admin landing alongside
the kebab overflow shortcut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/settings/settings_screen.dart

@@ -48,6 +48,7 @@ class SettingsScreen extends ConsumerWidget {
           _PasswordSection(),
           _Divider(),
           _ListenBrainzSection(),
+          _AdminSection(),
           SizedBox(height: 96),
         ],
       ),
@@ -87,6 +88,44 @@ class _SectionHeader extends StatelessWidget {
   }
 }
 
+/// Renders an "Admin" entry only when the current profile has
+/// `is_admin = true`. Returns an empty SizedBox otherwise so the
+/// const-children layout above stays valid.
+class _AdminSection extends ConsumerWidget {
+  const _AdminSection();
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final profile = ref.watch(_profileProvider).valueOrNull;
+    if (profile == null || !profile.isAdmin) return const SizedBox.shrink();
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        const _Divider(),
+        ListTile(
+          key: const Key('settings_admin_card'),
+          leading: Icon(Icons.shield, color: fs.parchment),
+          title: Text(
+            'Admin',
+            style: TextStyle(
+              color: fs.parchment,
+              fontFamily: 'Fraunces',
+              fontSize: 18,
+            ),
+          ),
+          subtitle: Text(
+            'Manage requests, quarantine, and users',
+            style: TextStyle(color: fs.ash),
+          ),
+          trailing: Icon(Icons.chevron_right, color: fs.ash),
+          onTap: () => context.push('/admin'),
+        ),
+      ],
+    );
+  }
+}
+
 class _ProfileSection extends ConsumerStatefulWidget {
   const _ProfileSection();
   @override