[US-02] Alternating shooting with feedback

Description

Implements alternating shooting system with hit/miss feedback for the Battleship game.

Changes

• Added turn-based shooting mechanics with proper player switching
• Implemented hit/miss detection and feedback system
• Added duplicate shot prevention
• Created comprehensive coordinate validation
• Added shooting service with extensive test coverage

Features

• Players take turns shooting at opponent's board
• Valid coordinate input (A1-J10 format)
• Hit/miss detection with appropriate messages
• Duplicate shot prevention with error messages
• Game state tracking (current player, game over conditions)

Acceptance Criteria Verified

✅ When firing at A1 for the first time, a message appears showing hit or miss, and that field becomes marked ✅ When the same field is targeted again, the shot does not go through, and the next player acts ✅ After one shot cycle finishes, the control changes so that only the other player can continue

Technical Implementation

• Game.java: Main game controller with turn management
• Board.java: Board management with shot tracking
• ShootingService.java: Coordinate validation and shooting logic
• Comprehensive test coverage with 20+ test cases

Closes #2