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/test/features/auth/login_screen_test.dart

@@ -0,0 +1,115 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:go_router/go_router.dart';
+
+import 'package:food_ai/core/auth/auth_provider.dart';
+import 'package:food_ai/features/auth/login_screen.dart';
+
+class _TestAuthNotifier extends AuthNotifier {
+  _TestAuthNotifier(super.initialState) : super.test();
+
+  @override
+  Future<void> signInWithEmail(String email, String password) async {}
+
+  @override
+  Future<void> signInWithGoogle() async {}
+
+  @override
+  Future<void> register(String email, String password, String name) async {}
+
+  @override
+  Future<void> signOut() async {}
+}
+
+Widget _buildTestWidget({AuthState? initialState}) {
+  final authState = initialState ??
+      const AuthState(status: AuthStatus.unauthenticated);
+
+  final router = GoRouter(
+    initialLocation: '/auth/login',
+    routes: [
+      GoRoute(
+        path: '/auth/login',
+        builder: (_, __) => const LoginScreen(),
+      ),
+      GoRoute(
+        path: '/auth/register',
+        builder: (_, __) =>
+            const Scaffold(body: Text('Register Screen')),
+      ),
+      GoRoute(
+        path: '/home',
+        builder: (_, __) =>
+            const Scaffold(body: Text('Home Screen')),
+      ),
+    ],
+  );
+
+  return ProviderScope(
+    overrides: [
+      authProvider.overrideWith(
+        (ref) => _TestAuthNotifier(authState),
+      ),
+    ],
+    child: MaterialApp.router(routerConfig: router),
+  );
+}
+
+void main() {
+  testWidgets('renders email and password fields and buttons',
+      (tester) async {
+    await tester.pumpWidget(_buildTestWidget());
+    await tester.pumpAndSettle();
+
+    expect(find.text('Email'), findsOneWidget);
+    expect(find.text('Пароль'), findsOneWidget);
+    expect(find.text('Войти'), findsOneWidget);
+    expect(find.text('Войти через Google'), findsOneWidget);
+    expect(find.text('Нет аккаунта? Регистрация'), findsOneWidget);
+  });
+
+  testWidgets('validates empty email', (tester) async {
+    await tester.pumpWidget(_buildTestWidget());
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Войти'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Введите email'), findsOneWidget);
+  });
+
+  testWidgets('validates invalid email format', (tester) async {
+    await tester.pumpWidget(_buildTestWidget());
+    await tester.pumpAndSettle();
+
+    await tester.enterText(
+        find.widgetWithText(TextFormField, 'Email'), 'abc');
+    await tester.tap(find.text('Войти'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Некорректный email'), findsOneWidget);
+  });
+
+  testWidgets('validates short password', (tester) async {
+    await tester.pumpWidget(_buildTestWidget());
+    await tester.pumpAndSettle();
+
+    await tester.enterText(
+        find.widgetWithText(TextFormField, 'Email'), 'test@test.com');
+    await tester.enterText(
+        find.widgetWithText(TextFormField, 'Пароль'), '123');
+    await tester.tap(find.text('Войти'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Минимум 6 символов'), findsOneWidget);
+  });
+
+  testWidgets('shows FoodAI branding', (tester) async {
+    await tester.pumpWidget(_buildTestWidget());
+    await tester.pumpAndSettle();
+
+    expect(find.text('FoodAI'), findsOneWidget);
+    expect(find.text('Управляйте питанием\nс помощью AI'), findsOneWidget);
+  });
+}