[2pts] Add leaderboard feature

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Description

Implement score calculation and leaderboard display functionality.

Depends on: SqliteGameRepository issue

Score Calculation

Calculate final score based on:

• Enemies defeated (points per enemy)
• Gold collected
• Level reached
• Bonus multipliers for difficulty

Leaderboard Display

• Show top 10 scores
• Display: rank, player name, score, date
• Accessible from main menu
• Friendly message if no scores exist

Integration

• Save score via IGameRepository.SaveScore() at game end
• Retrieve scores via IGameRepository.GetLeaderboard()

Acceptance Criteria

• Score calculation implemented based on metrics
• Considers enemies defeated
• Considers gold collected
• Considers level reached
• Score saved via IGameRepository.SaveScore() at game end
• Leaderboard accessible from main menu
• Displays top 10 scores
• Shows rank, name, score, and date
• Friendly message shown if no scores exist
• Properly formatted display output