Upgrade Next.js to 16.4.3 and pin Node 24

Next 16.2.6 carried ~20 advisories fixed in 16.4.x, including
unauthenticated RCE in the Image Optimization API (GHSA-2xp9-vwfh-vxw4,
GHSA-2464-fhr4-w9xw) and SSG/ISR cache poisoning (GHSA-mcj8-r9mp-w47p).
npm audit fix clears the remaining sharp/postcss/source-map-js/nanoid
findings; npm audit now reports 0 vulnerabilities.

Node 20 is EOL and receives no fix for Node2Shell (CVE-2026-111497,
bundled Brotli decoder OOB write). Pin .nvmrc and render.yaml to Node 24
LTS; Amplify SSR compute runs the major version used at build time.

README.md

@@ -108,7 +108,7 @@ works while in stub mode. The report is at `/report?cid=1&cs=anything`.
 The repo ships with `amplify.yml` for [AWS Amplify Hosting](https://aws.amazon.com/amplify/)
 (see `AMPLIFY_DEPLOY.md` for a first-time walkthrough) and `render.yaml`
 for [Render](https://render.com) (see `DEPLOYMENT.md`). The app is a
-standard Next.js 16 App Router project and runs anywhere Node 20+ can
+standard Next.js 16 App Router project and runs anywhere Node 24+ can
 run `next start` — Vercel, App Runner, Fly, self-hosted, etc.
 
 Build + start: