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 <[email protected]>

.claude/settings.local.json

@@ -0,0 +1,8 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(code .:*)",
+      "Bash(git add:*)"
+    ]
+  }
+}