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/src/repositories/mongo/models/UserModel.ts

@@ -21,7 +21,7 @@ const UserSchema = new Schema<
       lowercase: true,
       trim: true,
     },
-    displayName: {
+    userName: {
       type: String,
       required: true,
       trim: true,