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.

render.yaml

@@ -17,7 +17,7 @@ services:
       - key: NODE_ENV
         value: production
       - key: NODE_VERSION
-        value: "20"
+        value: "24"
       # Set these manually in the Render dashboard (UI → Environment).
       # Marked sync:false so they're not echoed in this committed file.
       - key: CIVI_BASE_URL