Make rendered markdown task list checkboxes clickable

[albcp]

Summary

Implements the feature request to allow rendered Markdown task list checkboxes to be clicked without entering edit mode. (https://js.wiki/feedback/p/allow-checkboxes-to-be-checked-without-editing)

Checkbox toggles are visual-only and are not persisted back to the Markdown source. This is useful for temporary checklist workflows, such as filling out a template before printing.

Changes

• enable interactive task list checkboxes in the server Markdown renderer
• enable the same behavior in the Markdown editor preview
• update task list checkbox styling so it no longer depends on disabled
• add a focused renderer test to verify disabled is not emitted

Verification

1. Create or edit a Markdown page with:
   • unchecked task
   • checked task
2. Save the page
3. View the rendered page
4. Click the checkboxes and confirm they can be toggled visually

Test

npx jest server/test/modules/rendering/markdown-tasklists.test.js

Screenshot

Rendered Markdown task list checkboxes are now clickable without entering edit mode.