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/_navbar.erb

@@ -16,10 +16,7 @@
       <span class="navbar-text me-2">Brother:</span>
       <div class="d-flex gap-1">
         <% BROTHER_PRINTERS.each do |key, info| %>
-          <form action="/brother/select" method="POST" class="m-0">
-            <input type="hidden" name="printer" value="<%= key %>">
-            <button type="submit" class="btn btn-sm <%= session[:brother_printer] == key ? 'btn-primary' : 'btn-outline-secondary' %>"><%= info[:label].sub('Brother ', '') %></button>
-          </form>
+          <a href="/brother/<%= key %>/" class="btn btn-sm <%= request.path.start_with?("/brother/#{key}/") ? 'btn-primary' : 'btn-outline-secondary' %>"><%= info[:label].sub('Brother ', '') %></a>
         <% end %>
       </div>
     </div>