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/router/app_router.dart

@@ -0,0 +1,93 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+
+import '../auth/auth_provider.dart';
+import '../../features/auth/login_screen.dart';
+import '../../features/auth/register_screen.dart';
+import '../../features/home/home_screen.dart';
+import '../../features/products/products_screen.dart';
+import '../../features/menu/menu_screen.dart';
+import '../../features/recipes/recipes_screen.dart';
+import '../../features/profile/profile_screen.dart';
+
+final routerProvider = Provider<GoRouter>((ref) {
+  final authState = ref.watch(authProvider);
+
+  return GoRouter(
+    initialLocation: '/home',
+    redirect: (context, state) {
+      final isLoggedIn = authState.status == AuthStatus.authenticated;
+      final isAuthRoute = state.matchedLocation.startsWith('/auth');
+
+      if (authState.status == AuthStatus.unknown) return null;
+      if (!isLoggedIn && !isAuthRoute) return '/auth/login';
+      if (isLoggedIn && isAuthRoute) return '/home';
+      return null;
+    },
+    routes: [
+      GoRoute(
+        path: '/auth/login',
+        builder: (_, __) => const LoginScreen(),
+      ),
+      GoRoute(
+        path: '/auth/register',
+        builder: (_, __) => const RegisterScreen(),
+      ),
+      ShellRoute(
+        builder: (context, state, child) => MainShell(child: child),
+        routes: [
+          GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),
+          GoRoute(
+              path: '/products',
+              builder: (_, __) => const ProductsScreen()),
+          GoRoute(path: '/menu', builder: (_, __) => const MenuScreen()),
+          GoRoute(
+              path: '/recipes', builder: (_, __) => const RecipesScreen()),
+          GoRoute(
+              path: '/profile', builder: (_, __) => const ProfileScreen()),
+        ],
+      ),
+    ],
+  );
+});
+
+class MainShell extends StatelessWidget {
+  final Widget child;
+
+  const MainShell({super.key, required this.child});
+
+  static const _tabs = [
+    '/home',
+    '/products',
+    '/menu',
+    '/recipes',
+    '/profile',
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    final location = GoRouterState.of(context).matchedLocation;
+    final currentIndex = _tabs.indexOf(location).clamp(0, _tabs.length - 1);
+
+    return Scaffold(
+      body: child,
+      bottomNavigationBar: BottomNavigationBar(
+        currentIndex: currentIndex,
+        onTap: (index) => context.go(_tabs[index]),
+        items: const [
+          BottomNavigationBarItem(
+              icon: Icon(Icons.home), label: 'Главная'),
+          BottomNavigationBarItem(
+              icon: Icon(Icons.kitchen), label: 'Продукты'),
+          BottomNavigationBarItem(
+              icon: Icon(Icons.calendar_month), label: 'Меню'),
+          BottomNavigationBarItem(
+              icon: Icon(Icons.menu_book), label: 'Рецепты'),
+          BottomNavigationBarItem(
+              icon: Icon(Icons.person), label: 'Профиль'),
+        ],
+      ),
+    );
+  }
+}