feat: add Flutter web frontend with authentication and recipe management features

flutter/lib/core/api/api_client.dart

@@ -0,0 +1,37 @@
+import 'dart:io';
+import 'package:http/http.dart' as http;
+
+/// Platform-neutral HTTP client wrapping the internal API base URL.
+/// Base URL is read from the FLUTTER_API_URL_INTERNAL environment variable
+/// (set by Docker) or falls back to localhost for local development.
+class ApiClient {
+  final String baseUrl;
+  final http.Client _client;
+
+  ApiClient({http.Client? client})
+      : baseUrl = const String.fromEnvironment(
+          'API_BASE_URL',
+          defaultValue: 'http://localhost:8080',
+        ),
+        _client = client ?? http.Client();
+
+  Map<String, String> _headers({String? token}) => {
+        'Content-Type': 'application/json',
+        if (token != null) 'Authorization': 'Bearer $token',
+      };
+
+  Future<http.Response> get(String path, {String? token}) =>
+      _client.get(Uri.parse('$baseUrl$path'), headers: _headers(token: token));
+
+  Future<http.Response> post(String path, String body, {String? token}) =>
+      _client.post(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token), body: body);
+
+  Future<http.Response> put(String path, String body, {String? token}) =>
+      _client.put(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token), body: body);
+
+  Future<http.Response> delete(String path, {String? token}) =>
+      _client.delete(Uri.parse('$baseUrl$path'),
+          headers: _headers(token: token));
+}