Checking for updates always dialed through the local proxy, which fails
exactly when an update may be needed most - when no working server is
available. Like the subscription-update menu, updating without the proxy
is now possible: the Check Update window gets an "Update via proxy"
toggle (persisted, on by default so existing behavior is unchanged),
honored by the check-only, update and geo-file flows, and by the
scheduled update check.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Introduce a new "Check Only" feature: add CheckOnlyCmd to CheckUpdateViewModel with CheckOnlyTask that queries updates (via UpdateService.CheckHasUpdateOnly) for selected cores and reports results without performing updates. Wire up a new btnCheckOnly in both Desktop and Avalonia views and bind the command. Add localized menuCheckOnly entries to multiple .resx files and update ResUI.Designer. Also shorten the pre-release label to "Check for pre-release" in resource files.
