v1.2.3

Fix nested checklist items rendering as plain bullets

The block parser's task-list regex never allowed leading whitespace or
captured an indent (unlike Bullet/Numbered), so a checklist item nested
under another list item fell through to the bullet pattern and rendered
as a bullet with literal "[ ] text"/"[x] text" instead of a checkbox.
MdBlock.Task now carries an indent, and MarkdownText pads the row to
match — tap-to-toggle and Enter-to-continue already handled indentation
correctly, so only the renderer needed the fix.

Co-Authored-By: Claude <noreply@anthropic.com>