Add Google Play Games reactivation guide

- Comprehensive guide for enabling Google Play Games Services
- Step-by-step instructions for Google Play Console setup
- Single line change required to reactivate functionality
- Documentation of all involved files and dependencies

REACTIVATION_GUIDE.md

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+# Google Play Games Services Reactivation Guide
+
+Google Play Games Services have been implemented in the ZenTap app but are currently **disabled** via a feature flag to allow for future activation without code changes.
+
+## Current Status
+
+- ✅ **Implemented**: All Google Play Games functionality is fully coded
+- ✅ **Configured**: Android app is properly configured with permissions and dependencies  
+- ✅ **Integrated**: Game tracking and UI components are integrated
+- ❌ **Disabled**: Feature flag prevents execution of Google Play Games methods
+
+## How to Reactivate
+
+To enable Google Play Games Services, you only need to change **one line** of code:
+
+### Step 1: Enable the Feature Flag
+
+In `/lib/utils/google_play_games_manager.dart`, change line 17:
+
+```dart
+// Current (disabled):
+static const bool _isEnabled = false;
+
+// Change to (enabled):
+static const bool _isEnabled = true;
+```
+
+### Step 2: Configure Google Play Console
+
+Before enabling in production, you must:
+
+1. **Create/Update Google Play Console App**
+   - Set up your app in Google Play Console
+   - Configure OAuth consent screen
+   - Add your app's SHA-1 fingerprint
+
+2. **Create Achievements** (in Google Play Console):
+   - `achievement_first_bubble` - First Bubble Popped
+   - `achievement_100_bubbles` - 100 Bubbles Popped  
+   - `achievement_1000_bubbles` - 1000 Bubbles Popped
+   - `achievement_5000_bubbles` - 5000 Bubbles Popped
+   - `achievement_zen_master` - Zen Master (10+ min session)
+   - `achievement_speed_demon` - Speed Demon (1000 pts in 2 min)
+   - `achievement_perfect_session` - Perfect Session
+
+3. **Create Leaderboards** (in Google Play Console):
+   - `leaderboard_high_score` - High Score
+   - `leaderboard_zen_mode` - Zen Mode Duration
+   - `leaderboard_total_bubbles` - Total Bubbles Popped
+   - `leaderboard_longest_session` - Longest Session
+
+4. **Update Configuration**:
+   - Replace placeholder IDs in `google_play_games_manager.dart` with actual IDs from console
+   - Update `app_id` in `/android/app/src/main/res/values/strings.xml`
+
+### Step 3: Test and Deploy
+
+1. Test with internal testing track in Google Play Console
+2. Verify achievements and leaderboards work correctly
+3. Deploy to production
+
+## What Works When Disabled
+
+When disabled (`_isEnabled = false`):
+- ✅ App runs normally without any Google Play Games functionality
+- ✅ All game mechanics work as expected  
+- ✅ No crashes or errors related to Google Play Games
+- ✅ UI shows "disabled" message in settings
+- ✅ No network calls to Google Play Games services
+
+## What Works When Enabled
+
+When enabled (`_isEnabled = true`):
+- ✅ User can sign in/out of Google Play Games
+- ✅ Achievements unlock automatically during gameplay
+- ✅ Scores submit to leaderboards after each session
+- ✅ Players can view achievements and leaderboards
+- ✅ Progress tracking across game sessions
+
+## Files Involved
+
+- `/lib/utils/google_play_games_manager.dart` - Main service class
+- `/lib/ui/google_play_games_widget.dart` - UI component
+- `/lib/game/zentap_game.dart` - Game integration
+- `/lib/ui/settings_screen.dart` - Settings integration
+- `/android/app/build.gradle.kts` - Android dependencies
+- `/android/app/src/main/AndroidManifest.xml` - Android permissions
+- `/android/app/src/main/res/values/strings.xml` - App configuration
+
+## Dependencies
+
+The app includes the required dependency:
+```yaml
+dependencies:
+  games_services: ^4.0.2
+```
+
+No additional dependencies need to be added when reactivating.