feat: Add initial backend server with user authentication, session management, and playlist CRUD functionality.

lib/core/api/api_client.dart

@@ -0,0 +1,83 @@
+import 'package:dio/dio.dart';
+import 'dart:html' as html;
+
+/// API Client for communicating with the backend
+class ApiClient {
+  static final ApiClient _instance = ApiClient._internal();
+  factory ApiClient() => _instance;
+  
+  late final Dio _dio;
+  String? _token;
+
+  ApiClient._internal() {
+    _dio = Dio(BaseOptions(
+      baseUrl: _getBaseUrl(),
+      headers: {'Content-Type': 'application/json'},
+      connectTimeout: const Duration(seconds: 10),
+      receiveTimeout: const Duration(seconds: 10),
+    ));
+    
+    // Add interceptor for logging
+    _dio.interceptors.add(LogInterceptor(
+      requestBody: true,
+      responseBody: true,
+    ));
+  }
+
+  /// Get base URL (same origin for production)
+  String _getBaseUrl() {
+    // Use current origin for API calls
+    return '';
+  }
+
+  /// Set authentication token
+  void setToken(String? token) {
+    _token = token;
+    if (token != null) {
+      _dio.options.headers['Authorization'] = 'Bearer $token';
+      // Store in localStorage for persistence
+      html.window.localStorage['auth_token'] = token;
+    } else {
+      _dio.options.headers.remove('Authorization');
+      html.window.localStorage.remove('auth_token');
+    }
+  }
+
+  /// Get stored token from localStorage
+  String? getStoredToken() {
+    return html.window.localStorage['auth_token'];
+  }
+
+  /// Restore token from localStorage
+  void restoreToken() {
+    final storedToken = getStoredToken();
+    if (storedToken != null) {
+      setToken(storedToken);
+    }
+  }
+
+  /// Clear token
+  void clearToken() {
+    setToken(null);
+  }
+
+  /// GET request
+  Future<Response> get(String path) async {
+    return _dio.get(path);
+  }
+
+  /// POST request
+  Future<Response> post(String path, {dynamic data}) async {
+    return _dio.post(path, data: data);
+  }
+
+  /// PUT request
+  Future<Response> put(String path, {dynamic data}) async {
+    return _dio.put(path, data: data);
+  }
+
+  /// DELETE request
+  Future<Response> delete(String path) async {
+    return _dio.delete(path);
+  }
+}