Support horizontal aspect ratios

Currently, SnapReddit exports threads in 1:1, 9:16, 3:4, and 4:5 vertical formats, optimized for Instagram and mobile-first layouts.

However, many creators repurpose Reddit visuals across horizontal platforms — including X (Twitter), YouTube thumbnails, LinkedIn, and desktop presentations.

This feature will introduce horizontal aspect ratio support (16:9, 1.91:1, and possibly 1:1 hybrid), enabling creators to render and export Reddit thread visuals in landscape-friendly dimensions without distortion or manual cropping.