2.0.1: Fix Paypal Express admin menu duplicating and never cleaning up

install() inserted an AdminMenu row (uri: admin::paypal-express) without
checking whether one already existed, and uninstall() deleted by uri
admin::paypal (missing "-express"), which never matched any row - so
every install/uninstall cycle left an orphaned duplicate entry under
"Payment method" in the admin sidebar instead of cleaning it up. Guard
install() against re-inserting when the row already exists, and fix
uninstall()'s uri so it actually removes it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>