Guarantee Safe First Click & Improve Bomb Placement

[REM-moe]

What this PR changes

• Implements field generation in the Game class to guarantee the first click is never a bomb (standard Minesweeper behavior).
• Adds a new Field.withSafePosition() factory using an efficient O(n) shuffle algorithm for bomb placement, replacing the old do-while loop.
• (game.width, game.height, game.bombCount) instead of accessing game.field before the field is generated.

Why

• Prevents the user from losing on the first click, matching expected Minesweeper behavior.

Issues

• Fixes: guarantee first-click is not a bomb #19

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