recent: Simplify down arrow navigation.
recent: Simplify down arrow navigation.

Fixes #23331

Combine checks for last row between `vim_dowm` and `down_arrow`.

Check for the presence of unread counter in `set_table_focus`
directly so that it can be used in other cases as well. This is
especially useful when marking the last row in the table as read.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip