When hermes dashboard --skip-build runs across agent updates, mount_spa checked WEB_DIST.exists() once at server startup and mounted an immutable 404 handler if the build was missing. As a result, subsequent builds while the server was running continued to serve 404 "Frontend not built".
- Removes early static return in mount_spa().
- Moves WEB_DIST.exists() check dynamically into _serve_index() and serve_spa().
- Mounts /assets StaticFiles with check_dir=False.
- Adds unit test test_mount_spa_dynamic_web_dist_recheck in tests/hermes_cli/test_web_server.py.
Closes#82614