Extract heading-stripping logic into `extractEditorBody` — fixes a bug
where the regex failed to strip the title heading from newly created
notes (because `splitFrontmatter` left a leading newline that the
regex didn't account for).
Add a fast path in `doSwap` for empty body content: when the body is
empty (common case for new notes), skip the potentially-async
`tryParseMarkdownToBlocks` pipeline entirely and apply a single empty
paragraph block synchronously. This eliminates the multi-second delay
caused by BlockNote's async markdown parser initialization.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
