Change package name to GitHub-based format

📦 Package Rename: com.sage.sage → com.github.mystiatech.sage

✅ Changes:
- Updated applicationId in build.gradle.kts
- Updated namespace in build.gradle.kts
- Moved MainActivity.kt to new package structure
- Updated google-services.json placeholder
- Updated FIREBASE_SETUP.md with correct package name
- Updated README_FIREBASE.md with correct package name

🎯 Reason:
- GitHub-based package names don't require domain ownership
- Standard convention: com.github.username.appname
- Ready for Firebase setup without needing website registration

📋 Firebase Setup:
When setting up Firebase, use package name:
com.github.mystiatech.sage

✅ Build Status:
- APK builds successfully (63.3MB)
- All tests passing
- Ready for Firebase configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

android/app/README_FIREBASE.md

@@ -15,7 +15,7 @@ The current `google-services.json` file is a **PLACEHOLDER** and will **NOT** wo
 
 ### Package Name (must match):
 ```
-com.sage.sage
+com.github.mystiatech.sage
 ```
 
 Without the real Firebase configuration file, household sharing will not work across devices!