Wallet backup: add async progress, success state, and destination handling

[johnny9]

Code Gap

The wallet settings backup action calls the model backup method synchronously and only reports failure. On success the UI clears the error text, but it does not confirm the backup, show the destination, or offer a reliable way to open the saved location.

Work Needed

• Move wallet backup work off the QML/UI thread or otherwise add a non-blocking busy state.
• Disable duplicate backup submissions while a backup is in progress.
• Show a success state with the destination path after backup completes.
• Provide release-scoped open-file or open-folder behavior for the saved backup.
• Surface failures from both backup creation and open-location handling.
• Add coverage for success, failure, cancel, duplicate submit, and open-location failure.

Priority

Beta / P1 primary backup hardening.