Downgrade upload-artifact to v3 for GHES compatibility

actions/upload-artifact@v4 is not supported on GHES/Forgejo Actions. Using v3 for compatibility.

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

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

.forgejo/workflows/build.yml

@@ -26,7 +26,7 @@ jobs:
         run: hugo --minify
       
       - name: Upload build artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: hugo-build
           path: public/