ci: build frontend assets for release packages

[jialudev]

Summary

• Add a release workflow web job that builds Web UI static assets before package builds.
• Upload and download web/static-dist so release binaries embed the generated frontend.
• Source Node and pnpm versions from web/app/package.json through setup-node and Corepack.

Verification

• ruby -e 'require "yaml"; YAML.load_file("csgclaw/.github/workflows/release.yml"); puts "yaml ok"'
• git diff --check