chore(frontend): Upgrade Next.js 14 → 15.5 and React 18 → 19

- Next.js upgraded from 14.2.0 to 15.5.4
- React upgraded from 18 to 19.2.0
- react-dom upgraded from 18 to 19.2.0

Frontend server working correctly:
- Dev server starts successfully
- Pages compile without errors
- HTTP 200 responses on all routes

Next steps:
- Fix next.config.js warning (swcMinify is deprecated)
- Upgrade MUI packages
- Upgrade testing libraries

maternal-web/next-env.d.ts

@@ -1,5 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
+/// <reference path="./.next/types/routes.d.ts" />
 
 // NOTE: This file should not be edited
-// see https://nextjs.org/docs/basic-features/typescript for more information.
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.