fix(PdfViewer): 隐藏返回顶部按钮并调整缩放控件位置

移除返回顶部按钮以简化界面
将缩放控件位置从底部60px调整为20px，避免与其他元素重叠

 <!--
  * @Date: 2025-01-21
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-10-22 11:39:56
+ * @LastEditTime: 2025-10-22 12:17:03
  * @FilePath: /mlaj/src/components/ui/PdfViewer.vue
  * @Description: PDF预览组件 - 使用pdf-vue3库
 -->
@@ -17,7 +17,7 @@
                     :showProgress="true"
                     :progressColor="'#1989fa'"
                     :showPageTooltip="true"
-                    :showBackToTopBtn="true"
+                    :showBackToTopBtn="false"
                     :scrollThreshold="300"
                     :pdfWidth="`${Math.round(100 * zoomLevel)}%`"
                     :rowGap="8"
@@ -58,13 +58,13 @@
                 <van-button class="zoom-btn zoom-in-btn" type="default" icon="plus" round @click="zoomIn" />
                 <div class="page-jump" @click="focusPageInput">
                     <span class="page-display">{{ currentPage }}/{{ totalPages || 0 }}</span>
-                    <input 
+                    <input
                         ref="pageInputRef"
-                        class="page-input" 
-                        type="number" 
-                        :min="1" 
-                        :max="totalPages || 1" 
-                        v-model="pageInput" 
+                        class="page-input"
+                        type="number"
+                        :min="1"
+                        :max="totalPages || 1"
+                        v-model="pageInput"
                         @blur="handlePageInputBlur"
                         @keyup.enter="handlePageInputBlur"
                         v-show="isEditingPage"
@@ -634,7 +634,7 @@ const addPreventSaveListeners = () => {
 /* 缩放控制按钮样式 */
 .zoom-controls {
     position: fixed;
-    bottom: 60px;
+    bottom: 20px;
     left: 50%;
     transform: translateX(-50%);
     z-index: 100;