feat: bidirectional click-to-select between video overlay and track cards

Redesign track cards with inline features, remove separate features panel, clean up overlays

Reduce left sidebar by 50% and move track cards to right of video

ui: Make video frame full-width, move features beside track cards

feat: Add threat chat feature for Q&A about detected threats

feat: Continuous object tracking, speed estimation, and overlay syncing

Refactor frontend into modular ES modules in frontend/ folder