[US-05] Save, load, and resume a match

Implementation Summary

This merge request implements the complete save/load functionality for the Battleship game as specified in issue #5.

Requirements Implemented

✅ 1) Persist complete match state to file

• SaveLoadService handles serialization of all game state
• Saves board size, fleet layouts, shot history, sunk statuses, current turn, and game state
• Hidden information remains concealed from opponent after load

✅ 2) Load and restore exact game state

• GameSaveService restores complete game state from files
• Preserves shot history, visible cell states, sunk indicators, current turn
• Maintains game difficulty/mode and timer value

✅ 3) Validate file and version

• isValidSaveFile() method validates file integrity
• Handles missing/invalid/corrupted/incompatible files
• Reports errors without modifying current game state

✅ 4) Ensure idempotent round-trip

• Save immediately followed by load preserves all observable state
• Comprehensive tests validate round-trip consistency
• No state changes during save/load cycle

✅ 5) User file selection

• Service methods accept file path parameters
• Controller layer handles user file selection
• Flexible file location and naming

Acceptance Criteria Validated

✅ AC1: Given a mid-game state S, when the user saves and immediately loads that file, then the restored game equals S in all observable aspects

• Tested in testAcceptanceCriterion1_SaveLoadPreservesCompleteState()
• Validates shot history, visible states, sunk indicators, current turn

✅ AC2: Given a deliberately corrupted or incompatible file, when the user attempts to load it, then an error is shown and the current game remains unchanged

• Tested in testAcceptanceCriterion2_CorruptedFileHandling()
• Validates error handling and state preservation

✅ AC3: Given a finished game (win or loss), when it is saved and later loaded, then the game remains finished with the same outcome

• Tested in testAcceptanceCriterion3_FinishedGamePreservation()
• Validates finished state preservation

Technical Implementation

Core Classes:

• SaveLoadService: Main service for save/load operations
• GameSaveService: Enhanced service with validation and testing
• GameSaveData: Serializable data structure for game state
• PlayerSaveData: Serializable data structure for player state
• ShipSaveData: Serializable data structure for ship state

Key Features:

• Java serialization for efficient state persistence
• Comprehensive error handling for file operations
• Complete state preservation including shot history
• Validation of save file integrity
• Round-trip consistency guarantees

Test Coverage:

• Unit tests for individual service methods
• Integration tests for complete save/load workflow
• Acceptance criteria validation tests
• Edge case handling (corrupted files, finished games)

Closes #5