Hide guides overlay and clean up barcode placeholder in cover PDF

Guides (trim/safe-zone/spine lines) are design references only and
must not appear in the submitted PDF — hidden via display:none.
Barcode placeholder is now an empty white box so KDP can place the
ISBN barcode automatically without printing a label.

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

templates/cover.html

@@ -141,21 +141,11 @@
     }
 
     /* 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;
-      border: 1px solid rgba(200,168,75,0.3);
-      display: flex;
-      align-items: center;
-      justify-content: center;
-    }
-
-    .barcode-placeholder span {
-      font-size: 0.07in;
-      color: #aaa;
-      text-align: center;
-      line-height: 1.4;
     }
 
     /* Optional back cover artwork */
@@ -301,6 +291,7 @@
      * if you prefer a clean proof. KDP will print nothing outside the trim.
      */
     .guides {
+      display: none; /* design reference only — enable locally to check layout */
       position: absolute;
       inset: 0;
       pointer-events: none;
@@ -368,10 +359,8 @@
         <p class="back-tagline">
           „Die schönsten Welten sind nur<br>mit geschlossenen Augen zu erreichen."
         </p>
-        {# Leave this box empty — KDP will place the barcode here automatically #}
-        <div class="barcode-placeholder">
-          <span>Barcode<br>(KDP)</span>
-        </div>
+        {# Empty white box — KDP places the ISBN barcode here automatically #}
+        <div class="barcode-placeholder"></div>
       </div>
     </div>