[feat] electron-updater auto-update

[ikseong00]

Goal

Make Tokie auto-update itself in the background after a new GitHub Release is published.

Why

Without auto-update, users stay on whatever version they first downloaded forever. For a pet that grows over time, that means missing balance fixes, new features, bug fixes.

Acceptance

• electron-updater integrated in electron/main.ts (check for updates on app start + every N hours).
• When an update is available: download in background, then prompt user before installing (modal or speech bubble).
• On accept: app restarts into the new version.
• CI release pipeline emits latest-mac.yml / latest.yml alongside the binary so electron-updater can detect releases.

Blockers / dependencies

• Practically requires code signing (Issue macOS code signing + notarization #4). Unsigned builds can technically auto-update but it's fragile on macOS.

Hints

• electron-builder publishes the metadata yml when you use --publish always and publish is configured in build.
• Update server is the GitHub Release itself; no separate server needed.