refactor: implement Flutter best practices and proper architecture

- Create proper service layer with AuthService and FirebaseAuthService
- Implement UserModel for proper data representation
- Enhance AuthState with proper loading states and error handling
- Convert stateless widgets to stateful where appropriate
- Add proper form validation and user feedback mechanisms
- Implement comprehensive error handling and loading indicators
- Fix redirect logic in router for proper authentication flow
- Create theme system with light and dark themes
- Add shared components like LoadingIndicator
- Improve code organization following recommended architecture
- Add proper disposal of controllers and focus nodes
- Implement proper null safety handling

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/shared/widgets/loading_indicator.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class LoadingIndicator extends StatelessWidget {
+  const LoadingIndicator({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Center(
+      child: CircularProgressIndicator(),
+    );
+  }
+}