fix: hide UI elements when printing

Add @media print styles to hide EditorPanel, popups, and loader.
Only the preview iframe content is visible during print.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/App.vue

@@ -348,4 +348,26 @@ onUnmounted(() => {
   z-index: 0;
   opacity: 0;
 }
+
+/* Hide UI elements when printing */
+@media print {
+  #editor-panel,
+  #element-popup,
+  #page-popup,
+  .preview-loader {
+    display: none !important;
+  }
+
+  .preview-frame {
+    position: static !important;
+    margin-left: 0 !important;
+    transform: none !important;
+    width: 100% !important;
+    height: auto !important;
+  }
+
+  .preview-frame:not(:first-of-type) {
+    display: none !important;
+  }
+}
 </style>