feat: Integrate Firebase Authentication and Firestore

- Added Firebase configuration to Android and iOS projects.
- Created `google-services.json` and `GoogleService-Info.plist` for Firebase setup.
- Implemented `AuthService` for handling user authentication with Firebase.
- Updated `UserProvider` to manage user data with Firestore.
- Refactored `ProfileContent`, `LoginPage`, and `SignUpPage` to use the new authentication service.
- Removed the old `UserService` and replaced it with Firestore-based user management.
- Added Firebase options in `firebase_options.dart` for platform-specific configurations.
- Updated dependencies in `pubspec.yaml` for Firebase packages.

android/app/build.gradle.kts

@@ -1,5 +1,8 @@
 plugins {
     id("com.android.application")
+    // START: FlutterFire Configuration
+    id("com.google.gms.google-services")
+    // END: FlutterFire Configuration
     id("kotlin-android")
     // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugins.
     id("dev.flutter.flutter-gradle-plugin")