Post-merge review of PR #26 (rewire the editor onto the core MarkdownEditor
hook), plus the release bump.

- Fix: the video toolbar inserted `![Video](url)` markdown, which the renderer
  turns into a broken <img>. Insert the renderer-supported `<Video url="…">`
  component instead — matching the image-insert path and the
  insert_component/insert_video_component handlers.
- Reformat editor_forms_test.exs (stray trailing blank line left by the
  push_slug_events/2 test removal in PR #26 — was failing `format --check`).
- Bump to 0.2.1 and add the CHANGELOG entry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>