_web_ui_build_needed() in PR #14914 checked web_dir/"dist" as the
sentinel, but vite.config.ts sets outDir: "../hermes_cli/web_dist" so
the build output lands in hermes_cli/web_dist/, never in web/dist/.
The sentinel was therefore always missing → _web_ui_build_needed always
returned True → npm install + Vite build ran on every startup → OOM on
low-memory VPS persisted unchanged.
Fix: derive dist_dir as web_dir.parent / "hermes_cli" / "web_dist" so
the sentinel points to the actual build output directory.
Fixes#14898
