fix: preserve original aspect ratio for clip cells

[nthnerr]

Issue:

Clip tiles were rendered as uniform squares regardless of source video dimensions. This fix makes each tile respect the clip's original aspect ratio.

Changes:

• backend/app.py — added get_video_dimensions() using PyAV; included width/height in collect_scenes()
• frontend/src/types/domain.ts — added width/height to ClipItem
• frontend/src/hooks/useImportExport.ts — mapped width/height in parseInitialClips()
• frontend/src/components/clipsGrid/LazyClip.tsx — applied aspect-ratio inline style to clip wrapper

Before:

After:

[nthnerr]