UI: enlarge artwork previews, preserve image aspect ratio, add click-to-open full-size artwork modal

- Increased preview widget size by ~5% for Box/Title/Ingame artwork.
- Fixed preview rendering to scale images proportionally and center them (no horizontal stretching).
- Added clickable artwork previews with hand cursor.
- Added modal full-size artwork viewer with scrollbars and close button.
- Added graceful handling for missing/unreadable artwork files.