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/server/scripts/hash-password.js

@@ -0,0 +1,10 @@
+const bcrypt = require("bcrypt");
+
+const password = process.argv[2];
+
+if (!password) {
+  console.error("Usage: node scripts/hash-password.js <password>");
+  process.exit(1);
+}
+
+bcrypt.hash(password, 10).then((hash) => console.log(hash));