feat(blog): 正文编辑器工具栏 sticky 跟随滚动

position: sticky; top: 0; z-index: 10
工具栏吸附 viewport 顶部, 编辑长文滚动时格式按钮始终可见
sticky 范围限制在 .rich-editor 父容器内, 滚出编辑器自然消失

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/admin/RichTextEditor.vue

@@ -134,6 +134,9 @@ async function handleFileChange(e) {
   border-bottom: 1px solid #e5e7eb;
   background: #fafafa;
   border-radius: 4px 4px 0 0;
+  position: sticky;
+  top: 0;
+  z-index: 10;
 }
 
 .toolbar-btn {