fix: restore mouse-wheel scrolling in preview-as-text outputs (#8863)

## Summary

Restore mouse-wheel scrolling for read-only preview widgets (PreviewAny
plaintext and markdown modes), broken by the focus-gated wheel capture
in #6597.

## Changes

- **What**: Remove `disabled` attribute from read-only textareas (keep
`readonly`) so they can receive focus and capture wheel events. Add
`data-capture-wheel` and `tabindex` to WidgetMarkdown display div.
- **Root cause**: `disabled` elements cannot receive focus in browsers.
The focus-gated `wheelCapturedByFocusedElement()` from #6597 always
evaluated to false for disabled textareas, forwarding all wheel events
to the canvas.

## Review Focus

- Verify that removing `disabled` while keeping `readonly` does not
allow unintended editing
- Confirm `tabindex="0"` on the markdown display div does not cause
unexpected tab-order issues
- Ensure trackpad panning over unfocused widgets (#6523) still works
correctly

Fixes COM-14812

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8863-fix-restore-mouse-wheel-scrolling-in-preview-as-text-outputs-3076d73d365081719bf5e453235bb2b5)
by [Unito](https://www.unito.io)

src/extensions/core/previewAny.ts

@@ -53,7 +53,6 @@ useExtensionService().registerExtension({
         showValueWidget.options.hidden = true
         showValueWidget.options.read_only = true
         showValueWidget.element.readOnly = true
-        showValueWidget.element.disabled = true
         showValueWidget.serialize = false
 
         showValueWidgetPlain.label = 'Preview'
@@ -61,7 +60,6 @@ useExtensionService().registerExtension({
         showValueWidgetPlain.options.hidden = false
         showValueWidgetPlain.options.read_only = true
         showValueWidgetPlain.element.readOnly = true
-        showValueWidgetPlain.element.disabled = true
         showValueWidgetPlain.serialize = false
       }