When the Stripe checkout fails to start, the subscription page now
shows an inline error in the Business Nitrate card under the CTA
instead of a toast. When the post-payment activation fails, the toast
message is updated to point users to support@penpot.app.
The nitrate-form modal also passed a URI object to
build-nitrate-callback-urls while the underlying append-query-param
relied on lambdaisland's u/parse, which only accepts strings. Switched
to the local u/uri helper so both strings and URI records work, so
failures opened from the modal land on the subscription page.
