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/theme/app_theme.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+import 'app_colors.dart';
+
+ThemeData appTheme() {
+  return ThemeData(
+    useMaterial3: true,
+    colorSchemeSeed: AppColors.primary,
+    scaffoldBackgroundColor: AppColors.background,
+    appBarTheme: const AppBarTheme(
+      centerTitle: true,
+      elevation: 0,
+      backgroundColor: AppColors.surface,
+      foregroundColor: AppColors.textPrimary,
+    ),
+    bottomNavigationBarTheme: const BottomNavigationBarThemeData(
+      selectedItemColor: AppColors.primary,
+      unselectedItemColor: AppColors.textSecondary,
+      type: BottomNavigationBarType.fixed,
+    ),
+    inputDecorationTheme: InputDecorationTheme(
+      border: OutlineInputBorder(borderRadius: BorderRadius.circular(12)),
+      contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 12),
+    ),
+    elevatedButtonTheme: ElevatedButtonThemeData(
+      style: ElevatedButton.styleFrom(
+        minimumSize: const Size(double.infinity, 48),
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
+      ),
+    ),
+  );
+}