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.

views/brother_picker.erb

@@ -3,16 +3,6 @@
 
 <div class="list-group" style="max-width: 400px;">
   <% @printers.each do |key, info| %>
-    <form action="/brother/select" method="POST" class="mb-2">
-      <input type="hidden" name="printer" value="<%= key %>">
-      <button type="submit" class="btn <%= @selected == key ? 'btn-primary' : 'btn-outline-primary' %> w-100 text-start">
-        <%= info[:label] %>
-        <% if @selected == key %> (active) <% end %>
-      </button>
-    </form>
+    <a href="/brother/<%= key %>/" class="btn btn-outline-primary w-100 text-start mb-2"><%= info[:label] %></a>
   <% end %>
 </div>
-
-<% if @selected %>
-  <p class="mt-3"><a href="/brother/">Open <%= @printers[@selected][:label] %> &raquo;</a></p>
-<% end %>