Closes audit v2 #20.
* ClientVersionApi — GET /api/client/version returning
UpdateInfo(version, apkUrl, sizeBytes).
* UpdateRepository.getLatest() + isVersionNewer() free function
ported from Flutter's component-wise integer compare (handles
our date-style 2026.05.10.1 versions correctly, with a
branch-name fallback for non-numeric builds like dev/main).
* AboutCardViewModel surfaces a four-state UpdateCheckResult
(Idle/Latest/UpdateAvailable/Error).
* About card grows a "Check for updates" button + inline status
line. "Install vX.Y.Z" wiring is #25 (post-v1).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
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