[US-04] Playing alone against the computer

Implementation Summary

This merge request implements the single-player mode for the Battleship game, allowing players to play against computer opponents with different difficulty levels.

Features Implemented:

1. AI Interface: Created AI interface defining contract for computer opponents
2. Easy AI: EasyAI class that makes random moves and places ships randomly
3. Normal AI: NormalAI class with targeting strategy that shoots around hits
4. Comprehensive Tests: Acceptance criteria tests validating all requirements

Acceptance Criteria Validated:

✅ When single-player mode starts, the computer's fleet exists, is valid and hidden

• AI places ships correctly without overlaps
• Ships are within board boundaries
• Fleet configuration is hidden from player

✅ When the computer makes a move, its shot is at a coordinate not previously targeted

• Both EasyAI and NormalAI track shot history
• No duplicate shots allowed
• Valid coordinate selection

✅ In Normal mode, if a shot hits, following shots should usually try neighboring cells until that ship is sunk

• NormalAI uses targeting queue for adjacent coordinates
• Prioritizes shots around successful hits
• Falls back to random when targeting exhausted

Technical Implementation:

• AI Interface: Standardized contract for all AI implementations
• EasyAI: Random ship placement and shooting
• NormalAI: Advanced targeting with hit tracking
• Test Coverage: Comprehensive acceptance tests for all criteria

Files Modified:

• src/main/java/org/example/ai/AI.java - AI interface
• src/main/java/org/example/ai/EasyAI.java - Easy difficulty implementation
• src/main/java/org/example/ai/NormalAI.java - Normal difficulty with targeting
• src/test/java/org/example/ai/AIAcceptanceTest.java - Acceptance criteria tests
• src/test/java/org/example/ai/EasyAITest.java - Unit tests for EasyAI
• src/test/java/org/example/ai/NormalAITest.java - Unit tests for NormalAI

Closes #4