feat(web): render editable tables and make images clickable

- Top-level GFM tables render as an editable widget: click a cell to
  edit its Markdown source, Tab/Enter/arrows to navigate, toolbar to
  insert/delete rows and columns or switch to source
- Structural table edits are isolated undo steps; focus is restored
  after the widget is rebuilt
- Clicking an image moves the cursor to its syntax to reveal the source
- Renderer escapes raw HTML and filters dangerous URL schemes by default

web/src/demo/sample.md

@@ -53,7 +53,7 @@ func main() {
 
 ![示例图片](https://vuejs.org/images/logo.png)
 
-## 表格（M2 实现可视化编辑）
+## 表格（点击单元格直接编辑，Tab 跳格，悬停显示工具栏）
 
 | 功能 | 状态 |
 | --- | --- |