feat: implement Iteration 0 foundation (backend + Flutter client)

Backend (Go):
- Project structure with chi router, pgxpool, goose migrations
- JWT auth (access/refresh tokens) with Firebase token verification
- NoopTokenVerifier for local dev without Firebase credentials
- PostgreSQL user repository with atomic profile updates (transactions)
- Mifflin-St Jeor calorie calculation based on profile data
- REST API: POST /auth/login, /auth/refresh, /auth/logout, GET/PUT /profile, GET /health
- Middleware: auth, CORS (localhost wildcard), logging, recovery, request_id
- Unit tests (51 passing) and integration tests (testcontainers)
- Docker Compose setup with postgres healthcheck and graceful shutdown

Flutter client:
- Riverpod state management with GoRouter navigation
- Firebase Auth (email/password + Google sign-in with web popup support)
- Platform-aware API URLs (web/Android/iOS)
- Dio HTTP client with JWT auth interceptor and concurrent refresh handling
- Secure token storage
- Screens: Login, Register, Home (tabs: Menu, Recipes, Products, Profile)
- Unit tests (17 passing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/core/config/app_config.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/foundation.dart' show kIsWeb, defaultTargetPlatform, TargetPlatform;
+
+class AppConfig {
+  final String apiBaseUrl;
+
+  const AppConfig({required this.apiBaseUrl});
+
+  static AppConfig get development {
+    if (kIsWeb) {
+      return const AppConfig(apiBaseUrl: 'http://localhost:9090');
+    }
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+        return const AppConfig(apiBaseUrl: 'http://10.0.2.2:9090');
+      case TargetPlatform.iOS:
+        return const AppConfig(apiBaseUrl: 'http://localhost:9090');
+      default:
+        return const AppConfig(apiBaseUrl: 'http://localhost:9090');
+    }
+  }
+
+  static const production = AppConfig(
+    apiBaseUrl: 'https://api.food-ai.app',
+  );
+}