fix(updates): manual check on an RC ignores newer RCs #1447

Problem

On a release candidate, Check for Updates says "up to date" even when a newer RC is published.

checkLatestRelease (electron/update-checker.ts) reads releases/latest, which GitHub resolves to the latest stable release and never to a prerelease. Running 2.2.0-rc.1, it got v2.1.2, which is older, and answered "OpenScreen is up to date (2.2.0-rc.1)" while 2.2.0-rc.2 was live.

The startup self-update check (electron-updater) does find the newer RC, because the running version is a prerelease. So the two paths disagree, and the manual button is the one RC testers click.

Observed (v2.2.0-rc.1 → rc.2 e2e, 31/10/2026)

• macOS, rc.1 official DMG: the manual check reports up to date. The release-page fallback can never trigger on an RC for the same reason.
• Windows, rc.1 per-machine: the manual check reports up to date. The background check offered "Update to 2.2.0-rc.2" about 70 s after launch.

Expected

When the running version is a prerelease, the manual check also considers prereleases. Stable installs keep reading releases/latest.

Pointers

• electron/update-checker.ts:1: LATEST_RELEASE_API
• electron/main.ts checkForUpdates(): calls checkLatestRelease, and only asks electron-updater after a newer version was found