feat: add avatar upload staging endpoint for preview deployments

Configure NEXT_PUBLIC_AVUP_ENDPOINT environment variable for preview
builds to enable avatar uploads on preview sites using the staging worker.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: storywithoutend

.github/workflows/pages-deployment.yaml

@@ -48,6 +48,8 @@ jobs:
       - name: Build and Export with Profiler
         if: ${{ github.ref != 'refs/heads/main' }}
         run: pnpm build:preview && pnpm export
+        env:
+          NEXT_PUBLIC_AVUP_ENDPOINT: https://avatar-upload-staging.ens-cf.workers.dev
 
       - name: Generate sitemap
         if: ${{ github.ref == 'refs/heads/main' }}