Fixed a bug where clicking checkboxes in task lists would toggle the wrong
checkbox when a memo contained multiple sections with separate task lists.

The issue was that TaskListItem was counting tasks only within the immediate
parent list (ul/ol), but the toggleTaskAtIndex function counts all tasks
globally across the entire memo. This caused index misalignment.

Changes:
- Add containerRef to MemoContentContext for proper task scoping
- Pass memoContentContainerRef through context in MemoContent component
- Update TaskListItem to count all tasks within the container scope

This ensures task indices are calculated consistently with the markdown
manipulation logic, fixing checkbox toggling in complex multi-section memos.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>