修复中间预览区滚动和表格溢出空白页

- 中间预览区: 改为flex布局, #render-preview设flex:1+min-height:0实现滚动
- 表格溢出: 溢出路径中TABLE块优先用splitTableByRows按行拆分
  替代margin-top负值CSS裁切,避免续接页丢失表头和内容不可见
  第一个chunk与当前页已有内容(H2等)同行,剩余chunks独立成页

view/admin/post/phone_image.html

@@ -83,9 +83,11 @@
 
         .render-preview-area {
             width: 540px;
-            overflow: hidden;
+            display: flex;
+            flex-direction: column;
             background: #fff;
             flex-shrink: 0;
+            overflow: hidden;
         }
 
         .render-preview-area .preview-header {
@@ -94,10 +96,12 @@
             border-bottom: 1px solid #e8e8e8;
             font-size: 13px;
             color: #999;
+            flex-shrink: 0;
         }
 
         #render-preview {
-            min-height: 300px;
+            flex: 1;
+            min-height: 0;
             padding: 20px;
             box-sizing: border-box;
             overflow-y: scroll;