transcripts: switch segment panel to tabbed views
Replace the right-panel checkbox/toggle model with a tab-based segment panel.\n\nAdd tabs for Transcript (default), Audio, Screen, and per-segment markdown files.\nUse cached tab panes for instant switching, render markdown tabs via marked, and keep Screen decoding lazy by preparing frames only on first Screen-tab activation.\n\nUpdate the segment API payload to include md_files (stem -> content) so markdown tabs render from a single segment fetch.