Use or convert color profiles for thumbnail generation

### Is your feature request related to a problem? Please describe.
When viewing thumbnails of HDR images with HDR color profiles (such as P3, Rec.2020, AdobeRGB etc.) in copyparty, they appear dull and almost greyscale.

### Describe the solution you'd like
I'd like thumbnails to properly honor the color profiles of the original files. Two potential approaches:
1. Convert the image to the sRGB color profile before generating thumbnails, ensuring consistent and accurate display across devices.
2. Embed the original color profile into the thumbnail, allowing browsers with HDR support to render them in high quality.

### Describe alternatives you've considered
Currently, there's no good workaround—either accept the dull thumbnails or avoid using HDR images altogether. I've considered manual conversion into extra subfolders for quick viewing (cf. screenshots).

### Additional context

**HDR images saved in Display-P3 color space**

**SDR images saved in sRGB color space**