Add desktop release pipeline and updater

[h4yfans]

What changed

• replace the desktop release workflow with a manual CI release flow that uses UTC calendar versions and prepends manual notes to GitHub-generated release notes
• build and publish updater-compatible desktop artifacts for Windows, Linux, and separate macOS x64 and arm64 releases, including merged mac updater metadata
• add Electron updater IPC, main-process orchestration, and a settings UI so users can check, download, and install updates in-app

Why

The previous release workflow depended on tag pushes and root-level build paths that did not match the desktop package layout, and the app had no in-product update flow.

Impact

• maintainers can cut GitHub releases from CI with date-based versions like 2026.4.23
• desktop users can update from inside the app instead of going to the release page
• release notes now serve both the GitHub release and the in-app updater prompt

Validation

• pnpm ipc:check
• pnpm typecheck:desktop