CI: publish the builds as a download site, deploy via Coolify

The deployed image is now a small nginx site instead of a bare app
tree: the Linux app (with its runtime), a universal jar, the Android
APK, SHA256SUMS and a landing page showing the commit and build date.

- desktop: universalJar bundles the Skiko natives for Linux, macOS and
  Windows (Compose's own jar only carries the building OS's).
- Dockerfile: the build stage runs tests and builds every artifact;
  site/assemble.sh lays them out; the final stage is nginx:alpine.
- .ci: after the image is built, `main` calls the beta Coolify hook and
  `release` the production one; other branches only build.
- .dockerignore keeps local.properties and secrets out of the build.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

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