v2.11.3 — unified installer/update build step

Maintenance/refactor release. install.sh and post_update.sh now share a single deploy-frontend.sh for building (esbuild + Tailwind) and deploying the web UI — including the React fetch, cache-busting, and a build-verification guard. This prevents the two paths from drifting apart again, which is what caused the install-only frontend breakage in #105/#107. No functional change for existing installs beyond more robust updates.