add auth

lib/features/auth/data/datasources/auth_remote_datasource.dart

@@ -56,6 +56,64 @@ class AuthRemoteDataSource {
     }
   }
 
+  /// Login
+  ///
+  /// Authenticates user with phone number.
+  /// Requires existing session (CSRF token and Cookie).
+  /// Returns new session with user credentials.
+  ///
+  /// API: POST /api/method/building_material.building_material.api.auth.login
+  /// Body: { "username": "phone", "googleid": null, "facebookid": null, "zaloid": null }
+  ///
+  /// Response includes new sid and csrf_token for authenticated user.
+  Future<GetSessionResponse> login({
+    required String phone,
+    required String csrfToken,
+    required String sid,
+    String? password, // Reserved for future use
+  }) async {
+    try {
+      final response = await _dio.post<Map<String, dynamic>>(
+        '/api/method/building_material.building_material.api.auth.login',
+        data: {
+          'username': phone,
+          'googleid': null,
+          'facebookid': null,
+          'zaloid': null,
+          // Password field reserved for future use
+          // 'password': password,
+        },
+        options: Options(
+          headers: {
+            'X-Frappe-Csrf-Token': csrfToken,
+            'Cookie': 'sid=$sid',
+            'Content-Type': 'application/json',
+          },
+        ),
+      );
+
+      if (response.statusCode == 200 && response.data != null) {
+        return GetSessionResponse.fromJson(response.data!);
+      } else {
+        throw ServerException(
+          'Login failed: ${response.statusCode}',
+        );
+      }
+    } on DioException catch (e) {
+      if (e.response?.statusCode == 401) {
+        throw const UnauthorizedException('Invalid credentials');
+      } else if (e.response?.statusCode == 404) {
+        throw NotFoundException('Login endpoint not found');
+      } else {
+        throw NetworkException(
+          e.message ?? 'Failed to login',
+        );
+      }
+    } catch (e) {
+      throw ServerException('Unexpected error during login: $e');
+    }
+  }
+
   /// Get Cities
   ///
   /// Fetches list of cities/provinces for address selection.