- Add Thumbnail field to FPS_Editor_BaseLevelEntry (UTexture2D)
- Replace BaseLevel combo box with button that opens a selection popup
- Popup displays base levels in a 3-column grid with thumbnails (2:1 ratio)
- Validate/Cancel buttons to confirm or dismiss selection
- Gray placeholder shown for entries without thumbnail
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>