feat: Implement initial authentication, multi-environment support, and core app infrastructure.

lib/config/app_config.dart

@@ -0,0 +1,16 @@
+import 'package:flutter_dotenv/flutter_dotenv.dart';
+
+enum Flavor { local, demo, dev }
+
+class AppConfig {
+  static late Flavor flavor;
+
+  static String get envFileName => '.env.${flavor.name}';
+
+  static Future<void> init(Flavor f) async {
+    flavor = f;
+    await dotenv.load(fileName: envFileName);
+  }
+
+  static String get apiBaseUrl => dotenv.env['API_BASE_URL'] ?? '';
+}