New `PhotoSize::XLarge` variant sits between `Large` (2048px) and
`Full` (original). On-demand generated and disk-cached at
`_xlarge/<hash>.jpg`, same waterfall as `Large` (embedded RAW preview
→ ffmpeg → image crate). Sources below 4096px serve at native size.
Reduces decoded bitmap memory from ~192MB (48MP full) to ~64MB for
the mobile viewer's zoom tier.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
cameron
b9175e2718