Add Auth service and Login Page

frontend_splatournament_manager/lib/services/auth_service.dart

@@ -0,0 +1,37 @@
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+
+class AuthService {
+  static const String baseUrl = "http://10.0.2.2:3000";
+
+  Future<Map<String, dynamic>> register(String username, String password) async {
+    final response = await http.post(
+      Uri.parse('$baseUrl/register'),
+      headers: {'Content-Type': 'application/json'},
+      body: json.encode({'username': username, 'password': password}),
+    );
+
+    if (response.statusCode == 201) {
+      return json.decode(response.body);
+    } else {
+      final body = json.decode(response.body);
+      throw Exception(body['error'] ?? 'Registration failed');
+    }
+  }
+
+  Future<Map<String, dynamic>> login(String username, String password) async {
+    final response = await http.post(
+      Uri.parse('$baseUrl/login'),
+      headers: {'Content-Type': 'application/json'},
+      body: json.encode({'username': username, 'password': password}),
+    );
+
+    if (response.statusCode == 200) {
+      return json.decode(response.body);
+    } else {
+      final body = json.decode(response.body);
+      throw Exception(body['error'] ?? 'Login failed');
+    }
+  }
+}
+