Support guessing

[Btimes2]

While the app works perfectly, it is hard to use it to guess numbers. Please consider to make the following features:

• [ ] show possible moves in each cell, maybe with hover text (maybe this could be turned on and off)
• [ ] three buttons: save, undo and redo to save the current state, revert to the previous saved state, and revert to the next saved state
• [ ] automatically save the current state before and after solving
• [ ] automatically verify the cells after each "computational move" using the basic sudoku rules - verification is okay after I wrote in a new number, but maybe the same could be done after the computer made a move
• [ ] do not continue solving after getting any errors (detecting illegal moves)

[Sasszem]

Many of these changes would need big changes on the Rust side and also in the JS side.

Currently, it goes as follows:

• there's one global Board which only holds the numbers 0-9 (0 means empty)
• JS can read/write this board with get and set
• JS can run validation on this (i.e check for collisions)
• JS can check if a particular square has a collision
• JS can run the solving algorithm which writes the result back to the global board

The solving algorithm uses the assumption that we set each cell only once, and because of that we can't simply update the solving state per-move, as the player might not keep this constraint (and has really no reason to do so, we should allow edits). This means the only way to check possibilities is to re-generate them on every move. It's wasteful (but I think it's performance impact is not that huge, so we might just do it).

Save states should be possible in JS (even saving it to localstorage for cross-session), and implementing undo/redo should not be that hard. Of course if we make any edit to the board we have to discard all the redo states (but we should filter if that edit actually changed something).

Computer moves having checks should also be possible, but needs some refactoring on the rust side, as well as stopping there.

Currently I don't have much time for this project, but I hope to do most of these changes.