feat: implement user authentication with login and register

- Add login screen with email/username support
- Add register screen with email validation
- Implement AuthStore with expo-secure-store (native) / localStorage (web)
- Add X-User-Id header authentication (simple auth without JWT)
- Rename displayName to userName across codebase
- Add findByUserName() to UserRepository
- Check for existing email AND username on registration
- Add AuthButton component with shadow effect
- Add logout button to Header
- Add hash-password.js utility script for manual password resets
- Update CORS to allow X-User-Id header

apps/client/package.json

@@ -24,6 +24,7 @@
     "expo-image": "~3.0.10",
     "expo-linking": "~8.0.9",
     "expo-router": "~6.0.15",
+    "expo-secure-store": "^15.0.8",
     "expo-splash-screen": "~31.0.11",
     "expo-status-bar": "~3.0.8",
     "expo-symbols": "~1.0.7",