feat: add HDR image and video support
- Add HDR detection utilities for wide color gamut formats (HEIC, HEIF, WebP)
- Apply colorSpace attribute to image/video elements for HDR-capable files
- Update frontend components (AttachmentCard, PreviewImageDialog, AttachmentList)
- Expand backend thumbnail generation to support HEIC, HEIF, WebP formats
- Add Color-Gamut response headers to advertise wide gamut support
- Extend avatar MIME type validation for HDR formats
Supported formats:
- Images: HEIC, HEIF, WebP, PNG, JPEG
- Videos: MP4, QuickTime, Matroska, WebM (VP9 Profile 2)
Browser support:
- Safari 14.1+, Chrome 118+, Edge 118+
- Gracefully degrades to sRGB on unsupported browsers
5612fb8f41