Markdown checklist items (`- [ ]` / `- [x]`) were rendered as plain
bullet points instead of checkboxes. The client-side conversion pipeline
(marked -> generateJSON) dropped the checkbox because:
- tiptapExtensions did not register TaskList/TaskItem.
- marked emits `<ul><li><input type="checkbox">`, but TipTap's parseHTML
rules only match `ul[data-type="taskList"]` / `li[data-type="taskItem"]`
with the checked state in `data-checked`.
Register TaskList/TaskItem and add normalizeTaskLists() to rewrite marked's
checkbox markup into the format TipTap expects before generateJSON runs.
collaboration.ts now reuses markdownToTiptapJson() so update_page and
comments share the same conversion.
