Improve shelf image previews and timestamp layout

[brzvsk]

What changed

• When a file card points to an image, Space preview detects it through its Uniform Type Identifier and renders the original file as an image
• The preview uses ImageIO downsampling sized for the larger Retina pane instead of the shelf card Quick Look thumbnail
• The Image search filter now includes image files as well as native clipboard image items, while ordinary files stay excluded
• Image-file filtering uses macOS content-type detection, so formats such as HEIC, WebP, SVG, and RAW do not need a hard-coded extension list
• Combining Image and File filters still returns each matching item once
• Non-image files keep the existing generic file preview and Quick Look action
• Relative timestamps stay on one line instead of wrapping inside narrow cards

Testing

• Debug app target builds successfully with Xcode 26.6
• All 187 CopyCore tests pass, including native images, image files, mixed multi-file items, non-image files, text-plus-Image search, and Image-plus-File composition
• Installed and launched a locally signed Debug build using the existing bundle identifier
• git diff --check passes