[stackyard-release]

Summary

• Date Unreleased as 1.18.0 and add its compare link.
• Bump api/package.json to 1.18.0.
• Pin the demo image to 1.18.0 in render.yaml.

Testing
The changelog gate ran in release mode before this branch was pushed. Merging this tags the release and publishes the image.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.