chore: Add next-env.d.ts for Next.js 16 type safety

- Added Next.js type declarations
- Required for proper TypeScript support in Next.js 16

frontend/next-env.d.ts

@@ -0,0 +1,5 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/building-your-application/configuring/typescript for more information.