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/build.gradle.kts

@@ -7,7 +7,7 @@ plugins {
 }
 
 android {
-    namespace = "com.sage.sage"
+    namespace = "com.github.mystiatech.sage"
     compileSdk = flutter.compileSdkVersion
     ndkVersion = flutter.ndkVersion
 
@@ -21,8 +21,7 @@ android {
     }
 
     defaultConfig {
-        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
-        applicationId = "com.sage.sage"
+        applicationId = "com.github.mystiatech.sage"
         // You can update the following values to match your application needs.
         // For more information, see: https://flutter.dev/to/review-gradle-config.
         minSdk = flutter.minSdkVersion  // Firebase requires minSdk 21