fix: move auth.ts to src/ to match @/ path alias

- Moved apps/web/auth.ts to apps/web/src/auth.ts
- Updated import in src/lib/viewer.ts to use @/auth alias
- Fixes 'Module not found: Can't resolve @/auth' error
- This error was not caught in dev due to typescript.ignoreBuildErrors
  but caused production build failures and 500 errors on /api/auth/* endpoints

The @/* path alias resolves to src/*, so auth.ts must be in src/ directory.
Previously, /api/auth/csrf and other NextAuth endpoints returned 500 errors.

apps/web/src/lib/viewer.ts

@@ -1,4 +1,4 @@
-import { auth } from '../../auth'
+import { auth } from '@/auth'
 import { cookies, headers } from 'next/headers'
 import { verifyGuestToken, GUEST_COOKIE_NAME } from './guest-token'