Add Epson TM-T20II receipt printing (title/body/QR, no ESC/POS knowledge needed)

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/.

views/_navbar.erb

@@ -12,6 +12,9 @@
         <li class="nav-item">
           <a class="nav-link" href="/upload">Shipping labels</a>
         </li>
+        <li class="nav-item">
+          <a class="nav-link" href="/epson">Receipts</a>
+        </li>
       </ul>
       <span class="navbar-text me-2">Brother:</span>
       <div class="d-flex gap-1">