Address Brother QL printer by URL path instead of session cookie

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.

README.adoc

@@ -10,8 +10,8 @@ Unified UI for printing labels/stickers:
 * `/` -- upload a PDF and print it (ZPL)
 * `/upload` -- upload form
 * `/test-print` -- print the bundled test label
-* `/brother` -- pick which Brother QL printer to proxy to
-* `/brother/select` -- switch the active Brother QL printer
+* `/brother` -- list of Brother QL printers
+* `/brother/<printer>/*` -- proxied to that printer's brother_ql_web instance (UI and API alike)
 
 == Running