Fix smoke tests: variant detection, Windows deps, UI reachability

- Phase 14: detect UI vs standard variant from HTTP server contents
  instead of hardcoding --standard (fixes UI smoke failure)
- Windows smoke: install zip + coreutils (sha256sum) in MSYS2
  (fixes exit code 127)
- Phase 15: UI HTTP server reachability test — verifies root returns
  200 and /rpc accepts POST (skips gracefully for non-UI builds)
- Fix switch fallthrough in release.yml (same MSYS2 deps)