Add prettier (closes #1)

Install prettier globally beside wrangler and wrangler's peers, and
check it in the smoke test. prettier ships no distro package for
Ubuntu, so npm global is the standard install.

Model: deepseek-v4-pro

README.md

@@ -27,6 +27,7 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | corepack        | `npm install -g corepack`      | nvm LTS global bin             |
 | yarn, pnpm      | corepack, cache pre-populated  | `/usr/local/corepack`          |
 | wrangler        | `npm install -g wrangler`      | nvm LTS global bin             |
+| prettier        | `npm install -g prettier`      | nvm LTS global bin             |
 | Python (distro) | apt `python3`, `pip`, `venv`   | `/usr/bin/python3`             |
 | pyenv           | git tag                        | `/usr/local/pyenv`             |
 | Python (latest) | `pyenv install`                | `/usr/local/pyenv/versions/*`  |