Better support for large puzzles

[MatrixFrog]

On my laptop, you can get about 15x15 working pretty well, but you have to reduce the terminal font size pretty far. To make this a better experience, maybe:

• render squares smaller
• render the boundaries between squares smaller, using box drawing characters or something
• make the main area scrollable
• ???

[MatrixFrog]

The QUADRANT_INSIDE borders mentioned here might help. https://forum.ratatui.rs/t/made-a-little-sliding-puzzle/188/4 because then maybe we don't need the spacing in between squares.