Backup/restore progress UI

[sastraxi]

Progress bar

Created a reusable uilib/progress_bar.py from the NAM capture panel. Per-instance colour stops (HEAT_STOPS for NAM's fixed-duration capture, STEEL_STOPS for archiving) with matching label pairs, and tweaked padding / tick count to keep things looking sharp.

Backup / restore features

• modalapi/archive.py runs the (un)archive subprocess off of the UI thread with size-weighted progress
• ui/archive_panel.py shows a fullscreen modal with the archive progress visualized using the new progress bar.
• atomicity: instead of re-adding to the same archive, util/data-backup.sh builds a new one and swaps it in on success
• backups are now cancellable; restores remain uncancellable

Fixed

• no longer look at mtime to determine whether or not to restore individual files; always restore (removes -u)
• the archive is no longer purely additive; e.g. deleted pedalboards are deleted from the backup
• no longer uses sudo (all files are pistomp-owned) so that we can cancel the subprocess if desired
• button now vertically centres its label properly

[rreichenbach]

Looks good.