feat: add more details when scanning

When adding a folder, the "scanning" status isn't very detailed and can take a lot of time with big folders, it would be nice if it was a bit more granular.

cc: @ethanndickson

The issue names no files, tests, or entry points; begin by locating the Swift code that handles adding a folder and reports its “scanning” status. Clarify which scan stages and progress signals should be exposed; done means large-folder scans show the agreed granular status.

索引モデルが issue の本文から書いたものです。