Single-runner workflow: test on every push; on v* tags cross-compile tefterd
for all platforms, build the Linux desktop app, and publish a Gitea release
via gitea.com/actions/release-action. Drop the GitHub workflow; document the
runner requirement and manual macOS/Windows desktop builds in the README.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
