Add GFM task list checkbox rendering

[zalun]

Summary

• Render - [x] and - [ ] GFM task list syntax as HTML checkboxes instead of plain bullet points
• Add CSS styling for task list items (remove bullets, inline checkbox with text)
• Checkboxes are read-only since the app is a viewer

Test plan

• Open a markdown file containing - [x] and - [ ] task list items
• Verify checked items show a checked checkbox and unchecked items show an empty checkbox
• Verify checkbox and text appear on the same line
• Verify regular unordered lists still render normally with bullets

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