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/package.json

@@ -27,10 +27,10 @@
     "axios": "^1.12.2",
     "date-fns": "^4.1.0",
     "framer-motion": "^11.18.2",
-    "next": "14.2.0",
+    "next": "^15.5.4",
     "next-pwa": "^5.6.0",
-    "react": "^18",
-    "react-dom": "^18",
+    "react": "^19.2.0",
+    "react-dom": "^19.2.0",
     "react-hook-form": "^7.63.0",
     "react-markdown": "^10.1.0",
     "react-redux": "^9.2.0",