Enhance: Integrate Google Play Games Services for global leaderboard

- Added global leaderboard functionality via Google Play Games Services.
- Implemented sign-in process for Google Play Games and score submission.
- Updated UI to include leaderboard buttons in both MainActivity and HighScoresActivity.
- Enhanced README to document new online features and leaderboard integration.
- Added necessary dependencies in build.gradle and updated AndroidManifest.xml for Google Play Games configuration.

README.md

@@ -11,6 +11,7 @@ A modern falling block puzzle game for Android, featuring smooth animations, res
 - Hard drop and soft drop controls
 - Advanced move detection (e.g., T-Spins) and scoring
 - Persistent high score system (Top 5)
+- Global leaderboard via Google Play Games Services
 
 ### Modern Android Features
 - Optimized for Android 11+ (API 30+)
@@ -20,6 +21,7 @@ A modern falling block puzzle game for Android, featuring smooth animations, res
 - Automatic Dark theme support
 - Intuitive and responsive touch controls
 - Full edge-to-edge display utilization
+- Google Play Games Services integration for leaderboards
 
 ### Scoring System
 
@@ -74,6 +76,12 @@ Pixelmint Drop features a comprehensive scoring system designed to reward skillf
 - Multiple theme options with ability to change manually or enable Random Mode (unlocked when 2+ themes are available).
 - In Random Mode, themes change automatically every 10 line clears (1 level).
 
+### Online Features
+- Global leaderboard support through Google Play Games Services
+- Sign in with your Google account to compete with players worldwide
+- Your highest scores are automatically submitted to the leaderboard
+- View the global leaderboard directly from the game
+
 ## Technical Details
 
 ### Requirements