fix: remove extra blank lines between list items on clipboard copy

[j4rviscmd]

Summary

When copying content from the editor (Ctrl+C) and pasting into other text editors, nested list items had extra blank lines between them. This was inconsistent with the Markdown export behavior which correctly produced tight lists.

Root Cause

BlockNote's clipboard handler uses remark-stringify to produce the text/plain clipboard data, which defaults to loose lists (blank lines between items). The Markdown export path already had tightenList() to fix this, but the clipboard copy path did not.

Solution

Added a useClipboardTightenList hook that post-processes the text/plain clipboard data after BlockNote writes it. The hook uses a conservative algorithm that only removes blank lines where both adjacent lines are list items, preserving intentional spacing between paragraphs and lists.

Changes

• src/features/editor/hooks/useClipboardTightenList.ts — New hook with clipboard-specific list tightening logic
• src/features/editor/ui/Editor.tsx — Register the new hook

Testing

1. Create a checklist with parent (x1) + children (x2+)
2. Select all and Ctrl+C
3. Paste into another text editor
4. ✅ No extra blank lines between list items
5. ✅ Blank lines between normal text and lists are preserved