Add 'Check Only' update action

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.