fix(transcripts): drop duplicate screen tab and nav hint
- Suppress md_files["screen"] in the segment-content route when screen
chunks are present, so the tab row no longer renders two screen tabs.
Speaker attribution still reads talents/screen.md from disk, unaffected.
- Lowercase the three structural JS tab labels (transcript / audio /
screen). Tab IDs and URL-hash routing are unchanged.
- Remove the [ ] to navigate hint span, its CSS, and its visibility
toggles. Keyboard navigation itself is unchanged.
Updates the segment-detail API baseline to match the intentional
contract change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>