Backport #40334 by @SudhanshuMatrix
Markdown renders `[X]` as a checked task like `[x]`, but parts of the
frontend only recognize lowercase `x`.
- Clicking a `[X]` checkbox now unchecks it instead of failing.
- Pressing Enter on a `- [X]` line in the editor continues the list with
`- [ ] ` instead of dropping the checkbox.
Assisted-by: Claude Code:claude-opus-5-5
Co-authored-by: Shudhanshu Singh <sudhanshuwriterblc@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
When a checkbox is toggled in the markup preview tab, the change is now
synced back to the editor textarea. Extracted a `toggleTasklistCheckbox`
helper to deduplicate the byte-offset toggle logic.
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Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>