Ersätter getAuthHeaders() + auth() standalone med withAuth() wrapper
i alla route handlers. Auth() standalone fungerar inte korrekt i
Next.js 16 + NextAuth beta.28 pga async cookies() kompatibilitet.
withAuth() använder auth() i wrapper-form sa att request.auth
populeras direkt av NextAuth.
Pavaerkade filer: 27 route handlers + ny lib/with-auth.ts
refactor(admin): integrate router refresh after product updates in forms
fix(imports): update fetch paths for product creation and update in ReceiptImportClient