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/google-services.json

@@ -9,7 +9,7 @@
       "client_info": {
         "mobilesdk_app_id": "1:PLACEHOLDER:android:PLACEHOLDER",
         "android_client_info": {
-          "package_name": "com.sage.sage"
+          "package_name": "com.github.mystiatech.sage"
         }
       },
       "oauth_client": [],