fix: handle spawn errors in run_cmd; deb-package install order + offline-safe test

src/node/utils/run_cmd.ts:
  Without `proc.on('error', ...)` a spawn failure (e.g. ENOENT for a
  missing binary) is emitted as an unlistened 'error' event, which
  Node treats as an uncaught exception that bypasses the awaiting
  try/catch and kills the process. The .deb hits this on first boot
  because plugins.ts spawns `pnpm --version` for a startup log line
  and pnpm isn't a runtime dep — Etherpad logs "Starting" then
  immediately stops. Reject the promise on 'error' so the existing
  try/catch in the caller actually catches it.

packaging/scripts/postinstall.sh:
  chown /var/lib/etherpad/plugin_packages AFTER `cp -a` from the
  staged tree — `cp -a` preserves source (root) ownership and was
  re-rooting the directory we'd just chowned to etherpad. Same
  ordering the var symlink block already used.

packaging/test-local.sh:
  Run `CI=1 pnpm install --frozen-lockfile` before staging so the
  package is built from a fresh, lockfile-consistent tree (matches
  CI). Fixes spurious "Cannot find module 'X'" failures from stale
  local symlinks pointing at out-of-date pnpm store paths.

End-to-end test now passes: postinstall asserts pass, /health
returns 200, dpkg --purge cleans up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear