[US-04] Single-player mode vs. computer opponent

Implementation Summary

This merge request implements single-player mode against a computer opponent with the following features:

Core Requirements Implemented:

1. Single-Player Mode: Computer places a legal fleet automatically (no overlaps/out-of-bounds, standard sizes)
2. Computer Turn Logic: Computer selects valid untargeted coordinates and avoids repeats
3. Difficulty Options:
   • Easy: Random shots
   • Standard: After a hit, preferentially targets adjacent cells until ship is sunk

Key Components:

• ComputerPlayer class with automatic ship placement and shot generation
• SinglePlayerGameService extending base game service for single-player flow
• Difficulty enum for Easy/Standard difficulty levels
• Comprehensive test coverage for acceptance criteria

Acceptance Criteria Verified:

• ✅ AC1: Single-player game initializes with computer fleet that exists, is legal, and hidden
• ✅ AC2: Computer takes turns with untargeted coordinates (never repeats shots)
• ✅ AC3: Standard difficulty targets adjacent cells after hits until ship is sunk

Technical Implementation:

• Layered architecture following planning documents
• Model-View-Controller pattern
• Java 21 with Maven build system
• JUnit 5 test framework with JaCoCo coverage

Closes #4