feat: app

[storopoli]

• Detect error in "related" cells and render the conflict in color: red;.
• Buttons and reactivity to input values in cells
• New game button (deal with dangling related cells)
• add docs for the new buttons
• Block original cells from being changed by the user
  • Maybe with a new prop in the Cell that is populated during creation and is checked in the button when onclick
• Undo button (tricky, we might need to store the App's state "timeline")