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

package-lock.json

@@ -34,6 +34,7 @@
         "expo-image": "~3.0.10",
         "expo-linking": "~8.0.9",
         "expo-router": "~6.0.15",
+        "expo-secure-store": "^15.0.8",
         "expo-splash-screen": "~31.0.11",
         "expo-status-bar": "~3.0.8",
         "expo-symbols": "~1.0.7",
@@ -7681,6 +7682,15 @@
         "node": ">=10"
       }
     },
+    "node_modules/expo-secure-store": {
+      "version": "15.0.8",
+      "resolved": "https://registry.npmjs.org/expo-secure-store/-/expo-secure-store-15.0.8.tgz",
+      "integrity": "sha512-lHnzvRajBu4u+P99+0GEMijQMFCOYpWRO4dWsXSuMt77+THPIGjzNvVKrGSl6mMrLsfVaKL8BpwYZLGlgA+zAw==",
+      "license": "MIT",
+      "peerDependencies": {
+        "expo": "*"
+      }
+    },
     "node_modules/expo-server": {
       "version": "1.0.5",
       "resolved": "https://registry.npmjs.org/expo-server/-/expo-server-1.0.5.tgz",