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/src/Themes.tsx

@@ -28,7 +28,7 @@ const defaultLight: Theme = {
   confirmButton: "#22c55e",
   rejectButton: "#ef4444",
   disabledButton: "#ccc",
-  buttonText: "#fff",
+  buttonText: "#000000",
   textPrimary: "#000000",
   textSecondary: "#666",
   textMuted: "#888",