Custom ReceiptBuilder instead of the escpos gem -- its diacritics handling
needed the same manual UTF-8->CP852 encode call anyway, based on earlier
experiments in ~/test/epson/.
Drops /brother/select and the session-based "active printer" state:
the printer key now lives in the path (/brother/<printer>/*), so the
proxy is stateless and any path under it -- including brother_ql_web's
API, not just its UI -- is reachable without a prior "activation" step.