Crop a clip in the editor and export the region

[mintycw]

Ultrawide (21:9) recordings currently go to ganked.tv whole and get cropped there. The editor should let people drag a crop rectangle over the preview (aspect presets 16:9 / 21:9 / 4:3 / 9:16 / free) and have Save, Save as copy and Upload write only that region.

Scope:

• Crop overlay over the preview and fullscreen view, with preset pills and an output-size readout.
• Export through the bundled ffmpeg (crop filter + libx264, audio copied), trim and crop in one re-encode. Trim-only keeps the lossless stream copy.
• Upload crops locally first, then sends the result.
• macOS has no bundled ffmpeg: the crop controls show a "needs ffmpeg" state, like in-app playback does.
• The crop rectangle is editor state like the trim range, not persisted.
• ADR documenting why cropping happens at export, not at record time (ADR 0019 chose letterbox over crop for recordings).

Follows up on #51, which shipped trim only.