ci: downgrade node to v12

Author: daforster

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
         uses: actions/setup-node@v6
         with:
           check-latest: true
-          node-version: 24
+          node-version: 12
       - name: Check Javascript codestyle
         run: npm install && npm run format-check
         working-directory: client