On Windows systems using a Chinese GBK locale, `hermes update` could misreport the Web UI build as failed even when `npm run build` actually succeeded. The failure was caused by Python decoding captured npm output with the process locale inside a background subprocess reader thread. When npm emitted bytes such as `0x85`, decoding under GBK raised `UnicodeDecodeError`, and Hermes then surfaced a misleading "Web UI build failed" warning.
This change makes the npm install/npm ci path and the Web UI build step decode captured output explicitly as UTF-8 with `errors="replace"`. That keeps unexpected bytes from crashing output collection, preserves successful builds, and prevents false negatives during update on Windows.
The patch also adds regression tests that verify these subprocess calls always use explicit UTF-8 decoding with replacement semantics.
_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
