The step's "Open System Settings" button did nothing: it routed the
`x-apple.systempreferences:` deep link through `openExternal`, whose
allowlist only passes http(s) and remote-editor schemes, so the link was
silently dropped.

Adds a dedicated `openSystemSettings(pane)` path instead. The renderer
sends a known pane identifier, not a URL, and the main process maps it to
an app-fixed deep link and opens it directly — so the general link
allowlist stays locked down while this one trusted destination gets
through. Uses the post-Ventura Full Disk Access anchor.

Also reworks the step's copy to say why the permission is needed and that
it can be revoked once the import is done.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>