Use origin + pathname as libraryReturnUrl default

Editor assessment

The subject states the behavioral change precisely: the library return URL now defaults to origin plus pathname. The feat type fits a new default behavior, and the absent scope is acceptable for a small embedding concern. No body is present, which is acceptable since the subject fully defines the new rule, though a note on which hosts it affects would have added color. The PR reference is well-formed. Imperative mood, no trailing period, compact length. A precise, self-sufficient small-feature message that a reader can evaluate at a glance.