fix: clean up stale raw mounts before scanning in attach wizard

After an interrupted attach wizard, the raw mount stays behind,
causing the device to appear as "mounted" in scan results. Now the
scan button calls cancel first, which unmounts any stale raw mounts
that have no bind mount in fstab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/storage/attach_other.go

@@ -28,3 +28,6 @@ func FinalizeAttach(req AttachRequest, progress chan<- FormatProgress) (string,
 func CleanupRawMount(rawPath string) error {
 	return fmt.Errorf("storage attach is only supported on Linux")
 }
+
+// CleanupStaleRawMounts is a no-op on non-Linux platforms.
+func CleanupStaleRawMounts() {}