Feat: Doubleclick to toggle edit for Markdown (litegraph) (#6560)

## Summary

See https://github.com/Comfy-Org/ComfyUI_frontend/pull/6537, but for
litegraph widget.

Doesn't allow dragging the node through the rendered markdown yet, that
would be more complicated (DOMWidget complication)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6560-Feat-Doubleclick-to-toggle-edit-for-Markdown-litegraph-2a06d73d36508189bf6eedd7cdeba6db)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

src/lib/litegraph/src/LGraphCanvas.ts

@@ -2169,7 +2169,7 @@ export class LGraphCanvas
     }
   }
 
-  processMouseDown(e: PointerEvent): void {
+  processMouseDown(e: MouseEvent): void {
     if (
       this.dragZoomEnabled &&
       e.ctrlKey &&

src/renderer/extensions/vueNodes/widgets/composables/useMarkdownWidget.ts

@@ -53,14 +53,7 @@ function addMarkdownWidget(
   widget.inputEl = inputEl
   widget.options.minNodeSize = [400, 200]
 
-  inputEl.addEventListener('pointerdown', (event: PointerEvent) => {
-    if (event.button !== 0) {
-      app.canvas.processMouseDown(event)
-      return
-    }
-    if (event.target instanceof HTMLAnchorElement) {
-      return
-    }
+  inputEl.addEventListener('dblclick', () => {
     inputEl.classList.add('editing')
     setTimeout(() => {
       textarea.focus()