feat(release): add public beta packaging channel

[wgqqqqq]

Summary

• add stable, beta, and nightly release-channel resolution and version projection
• publish public beta desktop packages as GitHub prereleases with a mutable channel-beta updater feed
• support fork-owned beta downloads and updater endpoints while keeping stable release mirrors canonical
• keep public beta scope to Desktop and Installer packages; skip CLI and Relay publishing
• accept raw Tauri signing key wrappers in release signing helpers
• add release-channel tests, workflow coverage, and releasing documentation

Validation

• pnpm run check:github-config
• pnpm run test:release-packaging
• bash -n scripts/sign-release-assets.sh
• public beta workflow run on fork succeeded: https://github.com/wgqqqqq/BitFun/actions/runs/31491075322
• published prerelease: https://github.com/wgqqqqq/BitFun/releases/tag/v0.2.18-beta.1

Notes

The beta workflow was exercised on the fork and verified across macOS arm64/x64, Windows x64, and Linux arm64/x64. The upstream PR contains the reusable packaging/channel changes; repository secrets remain fork-specific.