feat(auth): implement viewmodels for auth flow (MVVM)

Author: hoanghaoz

src/lib/viewmodels/auth_viewmodels.dart

@@ -0,0 +1,89 @@
+import 'package:flutter/material.dart';
+
+/// Base class to handle shared loading state
+class BaseViewModel extends ChangeNotifier {
+  bool _isLoading = false;
+  bool get isLoading => _isLoading;
+
+  void setLoading(bool value) {
+    _isLoading = value;
+    notifyListeners();
+  }
+}
+
+class LoginViewModel extends BaseViewModel {
+  final emailCtrl = TextEditingController();
+  final passCtrl = TextEditingController();
+  bool obscurePass = true;
+
+  void togglePass() { obscurePass = !obscurePass; notifyListeners(); }
+
+  Future<bool> login() async {
+    if (emailCtrl.text.isEmpty || passCtrl.text.isEmpty) return false;
+    setLoading(true);
+    // Mock API call
+    await Future.delayed(const Duration(seconds: 1, milliseconds: 500));
+    setLoading(false);
+    return true; // Assume success
+  }
+}
+
+class RegisterViewModel extends BaseViewModel {
+  final nameCtrl = TextEditingController();
+  final emailCtrl = TextEditingController();
+  final passCtrl = TextEditingController();
+  final confirmPassCtrl = TextEditingController();
+  bool obscurePass = true;
+
+  void togglePass() { obscurePass = !obscurePass; notifyListeners(); }
+
+  Future<bool> register() async {
+    if (passCtrl.text != confirmPassCtrl.text) return false;
+    setLoading(true);
+    await Future.delayed(const Duration(seconds: 1, milliseconds: 500));
+    setLoading(false);
+    return true;
+  }
+}
+
+class ForgotPassViewModel extends BaseViewModel {
+  final emailCtrl = TextEditingController();
+
+  Future<bool> sendCode() async {
+    if (emailCtrl.text.isEmpty) return false;
+    setLoading(true);
+    await Future.delayed(const Duration(seconds: 1, milliseconds: 500));
+    setLoading(false);
+    return true;
+  }
+}
+
+class OtpViewModel extends BaseViewModel {
+  List<String> digits = List.filled(6, "");
+
+  void updateDigit(int idx, String val) { digits[idx] = val; notifyListeners(); }
+
+  Future<bool> verify() async {
+    if (digits.join().length < 6) return false;
+    setLoading(true);
+    await Future.delayed(const Duration(seconds: 1, milliseconds: 500));
+    setLoading(false);
+    return true;
+  }
+}
+
+class NewPassViewModel extends BaseViewModel {
+  final passCtrl = TextEditingController();
+  final confirmPassCtrl = TextEditingController();
+  bool obscurePass = true;
+
+  void togglePass() { obscurePass = !obscurePass; notifyListeners(); }
+
+  Future<bool> updatePassword() async {
+    if (passCtrl.text != confirmPassCtrl.text) return false;
+    setLoading(true);
+    await Future.delayed(const Duration(seconds: 1, milliseconds: 500));
+    setLoading(false);
+    return true;
+  }
+}