Redesign video-slide matching UI for smooth user experience

- Videos auto-shown on scan with title from manifest
- × button to remove a video from alignment/note generation
- Each video has a slide dropdown pre-filled with auto-suggested match
- + button to add more slide files per video (multi-part lectures)
- − button to remove extra slide files
- Status indicators (green check = aligned, hollow circle = pending)
- Existing mapping loaded on scan so users pick up where they left off
- Header row with column labels for clarity
- Subtle row dividers for visual separation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nodeeeeee