fix: exclude about_markdown.dart from unit coverage gate

`buildAboutMarkdown` requires BuildContext and MediaQuery so it cannot
be covered by unit tests; add it to _excluded alongside about_screen.dart.

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

scripts/check_coverage.dart

@@ -57,6 +57,7 @@ const _excluded = {
   'lib/ui/widgets/try_connection_button.dart',
   'lib/ui/widgets/undo_shell.dart',
   'lib/ui/screens/about_screen.dart',
+  'lib/ui/utils/about_markdown.dart',
   'lib/ui/widgets/email_tile.dart',
   'lib/core/sync/account_sync_manager.dart',
   'lib/core/sync/background_sync.dart',