fix: update koshien extension icons to proper sizes

- Updated koshien-small.png from 80x80 to 40x40 pixels for consistency
- Verified koshien.png maintains standard 600x372 size
- Improves visual consistency across extension icons

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: takaokouji