feat(ui): add a Cancel button to manual backups

[ishaanko]

A manual backup started from the Create Backup screen could not be stopped. The screen showed "Please wait" until the backup finished, and for a live world the runtime handed the screen a result stage that ignored cancel.

The operation screen now shows a Cancel button while a backup creation runs. Cancel stops the running stage. For a live world the runtime returns a result view whose cancel goes through the same handler as the world-exit toast: it interrupts the capture thread while the world is copied, and cancels the coordinator operation once dispatched. The coordinator already removes the private capture and the partial ZIP, interrupts the Git write and cleans its staging, and keeps a destination that had finished before the stop. The screen reports "Backup cancelled" instead of a failure and offers no retry.

The cancellation check moved into AsyncTasks so the toast and the screen share one definition; a unit test covers it. The runtime and screen changes need the Minecraft client, so they were verified by build only.

Made with Claude Fable 5.1 in Claude Code.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

Safe to merge.

Summary

This update adds cancellation support for manual live-world backups while keeping the operation result aligned with the coordinator’s actual outcome. A cancellation request is forwarded without cancelling the result view itself, so a backup that has already entered recording can still report its recorded result.

_{Reviews (3) · Last reviewed commit: "test(zip): prove a cancelled backup leav..."}