v2.11.2 — installer frontend build fix

Fixes two fresh-install failures (#107), now that v2.11.1 lets the container create:

• Blocked UI / MIME error — app.js was served as text/html and blocked by nosniff. The installer's frontend build still used Babel, but the project uses esbuild + src/index.jsx, so app.js was never built and nginx's SPA fallback returned index.html for it. The installer now builds via build.sh (esbuild + Tailwind), deploys the bundle + React + cache-busts the version, and fails loudly if the build didn't produce app.js (instead of leaving a silently-broken UI).
• Collector can't reach Proxmox host — host detection now defaults to the resolved IP rather than the bare hostname (which the LXC usually can't resolve), fixing Max retries exceeded (host='pve').

Existing broken installs can be repaired without reinstalling — see issue #107.