Initialize project and update portal port configuration

Set default portal port to 8081, fix Dart build issue in cart screen, and update setup documentation.

Co-Authored-By: Oz <oz-agent@warp.dev>

lib/services/app_config_service.dart

@@ -0,0 +1,47 @@
+import 'dart:convert';
+
+import 'package:firebase_core/firebase_core.dart';
+import 'package:flutter/services.dart';
+
+class AppConfigService {
+  AppConfigService._();
+
+  static final AppConfigService instance = AppConfigService._();
+
+  bool _initialized = false;
+
+  Future<void> initialize() async {
+    if (_initialized) return;
+
+    final configRaw =
+        await rootBundle.loadString('assets/config/runtime_config.json');
+    final Map<String, dynamic> config = json.decode(configRaw);
+
+    await Firebase.initializeApp(
+      options: FirebaseOptions(
+        apiKey: _required(config, 'firebase_api_key'),
+        appId: _required(config, 'firebase_app_id'),
+        messagingSenderId: _required(config, 'firebase_messaging_sender_id'),
+        projectId: _required(config, 'firebase_project_id'),
+        authDomain: _required(config, 'firebase_auth_domain'),
+        storageBucket: _required(config, 'firebase_storage_bucket'),
+        measurementId: _optional(config, 'firebase_measurement_id'),
+      ),
+    );
+
+    _initialized = true;
+  }
+
+  String _required(Map<String, dynamic> config, String key) {
+    final value = (config[key] ?? '').toString().trim();
+    if (value.isEmpty || value.startsWith('replace-with-')) {
+      throw StateError('Missing required runtime configuration: $key');
+    }
+    return value;
+  }
+
+  String? _optional(Map<String, dynamic> config, String key) {
+    final value = (config[key] ?? '').toString().trim();
+    return value.isEmpty ? null : value;
+  }
+}