Add src/lib/auth.ts helper for extracting user from Auth.js session

Implement task 5.2: Create getAuthUser() helper that calls auth() from
src/auth.ts and returns the authenticated user object or null if not
authenticated. Mark task 5.2 complete in tasks.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/user-accounts/tasks.md

@@ -27,7 +27,7 @@
 ## 5. Auth Middleware & Helpers
 
 - [x] 5.1 `[sonnet]` Create `proxy.ts` at project root: protect `/app/*` routes (redirect to `/login`), protect `/api/*` except `/api/auth/*` and `/api/health` (return 401), redirect authenticated users from `/login` and `/register` to `/app`
-- [ ] 5.2 `[haiku]` Create `src/lib/auth.ts` with `getAuthUser()` helper that extracts user from Auth.js session
+- [x] 5.2 `[haiku]` Create `src/lib/auth.ts` with `getAuthUser()` helper that extracts user from Auth.js session
 
 ## 6. User Settings API
 

src/lib/auth.ts

@@ -0,0 +1,6 @@
+import { auth } from "@/auth";
+
+export async function getAuthUser() {
+  const session = await auth();
+  return session?.user ?? null;
+}