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>

.gitignore

@@ -0,0 +1,18 @@
+# Firebase credentials
+backend/firebase-credentials.json
+client/android/app/google-services.json
+client/ios/Runner/GoogleService-Info.plist
+
+# Backend
+backend/.env
+
+# Client build artifacts
+client/build/
+client/.dart_tool/
+client/.flutter-plugins
+client/.flutter-plugins-dependencies
+
+# IDE
+.idea/
+.vscode/
+*.iml