Lock board generation to 9×9 with difficulty-based mine counts

[pekomon]

Background

• Difficulty presets currently change board width/height, which drives layout bugs and inconsistent spacing across platforms.
• Product requirement: always render a 9×9 grid and vary only the number of mines per difficulty.
• History, timer, and win detection logic assumes variable board sizes and needs to be normalized.

Proposal

1. Update Difficulty to store only a mineCount (and optional label resources) while keeping board dimensions fixed at 9×9.
2. Refactor board generation (newGame, generateBoard) and any dependent logic/tests to respect the fixed size.
3. Adjust UI copy, history dialog, and tests to reflect the new mine counts (e.g., Medium = 40 mines on 9×9).
4. Add regression tests ensuring the generator produces 9×9 boards for every difficulty and mine counts match expectations.

Acceptance Criteria

• All difficulties produce a 9×9 board in runtime and unit tests.
• Mine counts match the previous presets (Easy 10, Medium 40, Hard 99) and win conditions still trigger correctly.
• UI strings/history entries reference mine counts, not board size, where applicable.
• ./gradlew test and ./gradlew build succeed; update snapshots/docs if any metadata mentions board dimensions.