Offset greater than current maximum items (total/unread) should yield last page and not the first

[woj-tek]

• I have read this document: https://miniflux.app/opinionated.html#feature-request

Currently when setting offset to value greater than current maximum (either total items or unread items) it fallbacks to first page. I think it would be better to (at least) try to display last possible page.

(somewhat related/aftermath of #2707)