[US-04] Playing alone against the computer

As someone who plays, I would like to play without a second person, against an automated opponent, so that I can play solo.

The system is expected to provide such a single-player mode where the computer puts ships on the board in a correct way (not overlapping, within limits). When it is the computer’s turn, it should pick a coordinate not already used and shoot. There could be different difficulties such as Easy (random) and Normal (where the computer shoots near previous hits), but precise behavior can vary.

Acceptance references: – When single-player mode starts, the computer’s fleet exists, is valid and hidden. – When the computer makes a move, its shot is at a coordinate not previously targeted. – In Normal mode, if a shot hits, following shots should usually try neighboring cells until that ship is sunk.