feat: Add initial Flutter Android project setup, core API client with authentication token management, and authentication service.

android/app/build.gradle.kts

@@ -21,7 +21,7 @@ android {
 
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
-        applicationId = "com.example.xtremflow"
+        applicationId = "com.xtremflow.mobile"
         // 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