Better Markdown rendering and a tidier Open With menu.
- Switched the Markdown engine to swift-markdown (cmark-gfm). Rendering is now CommonMark- and GitHub-Flavored-Markdown-compliant, so tables, task lists, strikethrough, and autolinks render the way you'd expect on GitHub.
- Fixed the Open With list. No more duplicate Markdown Preview entries from old build copies, and unrelated apps that only claim a generic plain-text association no longer show up — only apps that actually edit Markdown are listed.