[US-05] Saving and restoring the game

Description

Implements game save and load functionality for the Battleship game.

Changes

• Added GameService with save/load methods using Java serialization
• Implemented GameState class to store complete game state
• Added comprehensive test coverage for save/load scenarios
• Handles file not found, corrupted files, and incompatible formats
• Preserves all game state including boards, ships, shots, current player, difficulty, and running time

Features

• Save Game: Serializes complete game state to file
• Load Game: Restores game state from file with validation
• Error Handling: Graceful handling of missing/corrupted files
• State Preservation: All visible and hidden parts remain consistent after loading
• File Selection: Player can choose file location and name

Acceptance Criteria Verified

✅ If the game is saved and loaded right away, every visible part and all internal values should be identical ✅ If a corrupted file is loaded, an error is displayed and the game remains the same ✅ If a completed game is saved and later loaded, it stays finished and waits for a new start

Technical Details

• Uses Java Object Serialization for persistence
• Validates loaded game state integrity
• Maintains hidden ship positions after loading
• Preserves game timing and difficulty settings

Closes #5