Add Native aspect ratio option for background-free export

[Hemkesh]

Summary

• Adds a "Native" option to the aspect ratio dropdown that matches the cropped video's actual aspect ratio
• When selected, the video fills the entire frame with no background/wallpaper visible — useful for clean exports without any surrounding padding
• Automatically sets padding to 0 when Native is selected
• Works with crop: if you crop a 16:9 video to a square region, Native will export at 1:1

Test plan

• Open the aspect ratio dropdown in the timeline and verify "Native" appears
• Select Native and confirm the preview adjusts to the video's actual aspect ratio with no background
• Crop the video to a different shape, select Native, and verify the preview/export matches the cropped dimensions
• Export with Native selected and confirm the output has no background padding
• Verify other aspect ratio presets still work as before

🤖 Generated with Claude Code