feat(settings): About section with version + force update check

Adds an About card to Settings that shows the installed version
(version+build from PackageInfo), the latest known version from
clientUpdateProvider, and a "Check for updates" button that
invalidates the provider to force a fresh poll. When an update is
available, surfaces an Install CTA that reuses the same installer
flow as the top banner.

The existing banner (shouldShowUpdateBannerProvider) is unaffected
— it gates on per-version dismissal, while the About section
always reflects the current provider state regardless of dismissal.

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

flutter_client/lib/settings/settings_screen.dart

@@ -9,6 +9,7 @@ import '../library/library_providers.dart' show dioProvider;
 import '../models/my_profile.dart';
 import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
+import 'about_section.dart';
 import 'storage_section.dart';
 import '../theme/theme_mode_provider.dart';
 
@@ -56,6 +57,8 @@ class SettingsScreen extends ConsumerWidget {
           _PasswordSection(),
           _Divider(),
           _ListenBrainzSection(),
+          _Divider(),
+          AboutSection(),
           _AdminSection(),
           SizedBox(height: 96),
         ],