feat(linux): package Linux release, deploy to server, add in-app update banner

Build task embeds GIT_HASH via --dart-define; new deploy-linux-to-server task
packages a tar.gz and updates latest.json on the server. The account list screen
shows a MaterialBanner when a newer Linux build is available.

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

scripts/check_coverage.dart

@@ -63,6 +63,7 @@ const _excluded = {
   'lib/data/repositories/sync_log_repository_impl.dart',
   'lib/data/repositories/undo_repository_impl.dart',
   'lib/data/repositories/search_history_repository_impl.dart',
+  'lib/core/services/update_service.dart',
 };
 
 void main() {