Update CI Node.js version to 18 for Astro v5 compatibility

Astro v5 requires Node.js >= 18.20.8, but the CI was using Node 16.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Toasterson

.github/workflows/deploy.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Install Node
         uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 18
 
       - name: Cache npm modules
         uses: actions/cache@v3