Install and publish Windows and Linux desktop releases #325

The release workflow only builds on macOS. release.ts, ci.ts and github-release.ts assume one macOS-arm64 DMG; publishing is Sparkle-only.

Done:

• Choose installer/package formats and installation scope; implement launch/menu integration, helper registration and uninstall cleanup while retaining user data.
• Build each supported OS/architecture in native CI, with target-bearing manifests, checksums and source revisions.
• Sign Windows releases and authenticate Linux artifacts/packages; keep signing credentials in CI.
• Publish coherent per-platform assets/downloads and update feeds without target collisions; never advertise an artifact before upload and verification. Preserve immutable archives, explicit publish, canary/stable isolation and retry safety.
• Include target-specific third-party notices and clean-machine installation evidence from Qualify Windows and Linux desktop support on real machines #326.

Payloads are #315; safe upgrades are #324. Reuse #277 for the installed CLI; existing macOS release work remains #217.

Part of #314.