RemoveLibrary now polls until the cancelled scan goroutine finishes
before proceeding with the removal transaction. Also show removal
errors as toast messages instead of only logging to console, and
explicitly reload library list after successful removal.
Tracks from pre-multi-library schema have library_id=0 (default).
When AddLibrary creates a new library row, UPDATE orphaned tracks
whose file_path falls under the library's directory to use the new
library_id. This prevents the scan from hitting UNIQUE constraints
on every file and ensures track counts are correct immediately.
yonlu