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

packages/shared/src/models/User.ts

@@ -1,7 +1,7 @@
 export interface User {
   id: string;
   email: string;
-  displayName: string;
+  userName: string;
   passwordHash?: string;
   createdAt?: Date;
   updatedAt?: Date;
@@ -9,12 +9,12 @@ export interface User {
 
 export interface CreateUserDTO {
   email: string;
-  displayName: string;
+  userName: string;
   password: string;
 }
 
 export interface LoginDTO {
-  email: string;
+  identifier: string;
   password: string;
 }