Add cancel button for running deployments

[Wiesenwischer]

Problem

There is no way to cancel a running product deployment from the UI. If a deployment gets stuck (e.g., due to SSH tunnel issues or image pull failures), the user has no option to abort it.

Expected Behavior

• A "Cancel" button should be available on the deployment progress page while a deployment is in progress
• Cancelling should stop remaining stack deployments and mark the deployment as cancelled
• Already completed stacks should remain running

Context

Discovered during SSH tunnel testing — a product deployment with 14 stacks got stuck on image pulling due to tunnel instability (v0.53.0 bug). No way to cancel from the UI.