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.

package.json

@@ -13,18 +13,18 @@
     "test": "node --test lib/staff-field-mapping.test.mjs lib/mime.test.mjs"
   },
   "dependencies": {
-    "next": "16.2.6",
+    "next": "16.4.3",
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "react-hook-form": "^7.75.0"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
-    "@types/node": "^20",
+    "@types/node": "^24",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "eslint": "^9",
-    "eslint-config-next": "16.2.6",
+    "eslint-config-next": "16.4.3",
     "tailwindcss": "^4",
     "typescript": "^5"
   }