ci: upgrade site build to Node 24

The runner's default Node 20 is no longer supported. Add explicit
setup-node step with Node 24 before setup-bun.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees

.github/workflows/site-ci.yml

@@ -19,6 +19,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '24'
       - uses: oven-sh/setup-bun@v2
       - run: bun install
       - run: bun run build