ux: allow interrupting in-progress push operation in dialog

[sdpfigueiredo]

Resolve #2603

Problem

When the remote server is unreachable (e.g. corporate VPN down), the Push Changes to Remote dialog blocks for minutes on the hung git push process with no way to interrupt it — the dialog is not closable while the operation is in progress.

Solution

Add an opt-in cancellation mechanism for popups and wire it up for the Push dialog:

• Popup base class gains CanCancel (default false) and Cancel(), and Cleanup() becomes virtual.
• Push overrides CanCancel => true and Cancel() → cancels a CancellationTokenSource whose token is passed to the push command via the existing .WithCancellation(token) extension. Command.ExecAsync() already kills the git process tree on cancellation, and returns true when cancelled — so the dialog closes itself once the operation is interrupted.
• PopupRunningStatus shows a CANCEL button next to the running indicator, visible only when the popup opts in (CanCancel).
• LauncherPage.CancelPopup() now forwards to popup.Cancel() when a cancellable popup is in progress, so ESC and mask-click also interrupt the running push.

Behavior

1. Push is running (hung on unreachable remote)
2. User clicks CANCEL (or presses ESC / clicks outside)
3. git push process tree is killed
4. Dialog closes immediately

Notes

• Changes are opt-in: no other dialog is affected (button only appears for popups with CanCancel == true).
• PushTag / PushRevision follow the same pattern and can opt in later if desired.

Tested: dotnet build clean (0 warnings / 0 errors). Windows win-x64 NativeAOT publish successful.

Screenshot