Improve puzzle builder

[PlayLikeNeverB4]

• Add toggle to show solution (if available)
• Add toggle to show all cell candidates (not useful to add given digits in single-candidate cells)
• Show which cells can already be deduced by intuitive solution (not useful to add given digits there)
• When brute solver returning multiple solutions - show which cells differ in the 2 solutions
• Thermo difficulty estimation is weirder, think of something that makes sense
• Enter = shortcut to add constraint
• Add puzzle - warning if user didn’t change difficulty