Add column-gap to list header/items, prevent rubber-band selection from
swallowing clicks on list headers and file items, preserve rubber-band
selection after mouseup, and extract type aliases to resolve clippy
type_complexity warnings.
Extract EXIF orientation, GPS coordinates, camera info, and timestamps
from uploaded images using kamadak-exif. Store metadata in a new
file_metadata PG table. Apply EXIF orientation to thumbnail generation
so images display correctly. Add /api/files/{id}/metadata endpoint.