Smoother Preview scrolling and reliable relative links

Fixed

• Smooth Preview scrolling on long code blocks. Preview now stays smooth while scrolling through notes with very long fenced code blocks, like transcripts or large JSON dumps, including when line numbers are enabled.
• Relative Markdown links open in Cogito. Cmd-clicking a relative Markdown link in Preview, like [transcript](folder/note.md), now opens that note inside Cogito instead of failing with a system error. Line fragments such as #L42 are preserved, and web links keep opening in your browser.