uSync.Complete: fix Publisher push Cancel not stopping unsaved-changes flow

Fixed Jumoo/uSync.Complete.Issues#308: pressing Cancel on the "unsaved changes" dialog when pushing a document to a remote server didn't actually cancel — the push-to-remote dialog opened anyway. Root cause was a control-flow bug in SyncPublisherDialogActionBase.execute() where the push-dialog code ran unconditionally regardless of the pending-changes modal's outcome
Replaced the 2-button dialog with three clear options — Cancel / Ignore / Save and Publish — each with a one-line explanation, and applied the same fix to the Schedule/Pull sub-menu action which shared the same (also-broken) dialog
Merged as PR #175 into v18/main