"Scanning for changes..." doesn't show progress

[Chetic]

"Scanning for changes..." takes a long time on some setups, such as when scanning samba mounts.
It would be very nice to get some indication of the progress it makes, or how far it has scanned as a progress bar.

[Chetic]

Fixed in 141ca1e. The scanning step now shows a live file count during the scan.

What changed:

1. step_update() method on IndexingUI — Updates the spinner message in-place while the spinner continues animating. The spinner thread already reads _step_message every 0.1s, so it picks up the new text automatically.

2. Original message preserved for step_done() — A new _step_original_message field stores the initial step_start() text. step_done() uses it for the final line, so the output reads Scanning for changes... 3 new rather than Scanning for changes (4,521 files)... 3 new.

3. Scanning counter in build_index() — Calls ui.step_update() every 100 files during the scan loop, showing e.g. Scanning for changes (4,500 files).