fix: remove barcode placeholder rectangle from back cover

KDP generates and places the ISBN barcode automatically during upload;
a manual white box in the PDF conflicts with that process.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/cover.html

@@ -211,13 +211,6 @@
       text-shadow: 0 1px 4px rgba(0,0,0,0.8);
     }
 
-    /* Barcode placeholder — KDP places the barcode here if left blank */
-    /* White box reserved for KDP's automatic ISBN barcode placement */
-    .barcode-placeholder {
-      width: 1in;
-      height: 0.6in;
-      background: white;
-    }
 
     /* Optional back cover artwork */
     .back-artwork {
@@ -493,8 +486,6 @@
           <p class="back-tagline">
             „Die schönsten Welten sind nur<br>mit geschlossenen Augen zu erreichen.“
           </p>
-          {# Empty white box — KDP places the ISBN barcode here automatically #}
-          <div class="barcode-placeholder"></div>
         </div>
       </div>
     </div>