[US-05] Saving and restoring the game

Description

Implements comprehensive save/load functionality for the Battleship game. Players can save the current game state to a file and later load it to continue from the exact same point.

Changes

• Created GameSaveState class for serialization with all required game data
• Made all model classes (Coordinate, Ship, Board, Player) serializable
• Added saveGame() and loadGame() methods to GameController
• Implemented error handling for corrupted/missing files
• Added running time tracking for game sessions

Requirements Met

✅ Store everything belonging to the match in a file ✅ Hidden things must stay hidden after loading ✅ Previous game state must be shown exactly as before ✅ Report errors for missing/damaged/incompatible files ✅ Leave running game unchanged if file loading fails ✅ Player can choose file location and name ✅ Save right before loading should not change match state

Acceptance Criteria

✅ If 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 state persistence
• Comprehensive error handling for file operations
• Preserves all game elements: ship placements, shots, turn order, game state
• Maintains hidden ship positions after loading

Closes #5 (closed)