Refactor Auth Module from GetX + MVC to Feature-Based MVVM with Riverpod

### 🚀 The feature

Refactor the existing authentication module from the current MVC architecture using GetX to a feature-based MVVM architecture powered by Riverpod.

### Motivation, pitch

The current implementation tightly couples business logic, state management, and UI concerns, making long-term maintenance and feature expansion more challenging. Migrating to a feature-based MVVM structure with Riverpod will provide clearer separation of concerns, better dependency management, improved testability, and a more scalable foundation for future development.