Video/Image Renders - use splat file name as render file name

[azadbal]

Summary

Use the first imported splat's filename as the suggested save name for rendered images and videos.

What changed

• derive render save names from the first scene splat instead of the document name or supersplat
• preserve the existing showSaveFilePicker flow so saving to macOS locations like Downloads and Documents continues to work
• keep the existing browser download fallback for environments without the file save picker
• share the render filename logic between image and video export paths

Notes

This PR intentionally keeps the save flow close to stock supersplat.
It updates the suggested filename, but does not add folder inspection or auto-incremented (1), (2) collision handling.

Validation

• npm run lint
• npm run build
• browser sanity check in Chrome: importing splat1.ply suggests splat1.png in the save picker
• manual verification that the same branch no longer hits the folder-access error caused by showDirectoryPicker