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/app.ts

@@ -35,7 +35,7 @@ if (process.env.NODE_ENV !== "production") {
       "Access-Control-Allow-Methods",
       "GET, POST, PUT, DELETE, OPTIONS",
     );
-    res.header("Access-Control-Allow-Headers", "Content-Type, Authorization");
+    res.header("Access-Control-Allow-Headers", "Content-Type, Authorization, X-User-Id");
     if (req.method === "OPTIONS") {
       res.sendStatus(200);
       return;