Release v2.15.4

fix(pi): SumUp cancel, failure sheet, and pay lock

Summary

• Operators can Abbrechen an in-progress SumUp connected checkout from the shared waiting overlay (all POS entry points).
• Failed/declined/timed-out SumUp attempts show a prominent failure sheet (not toast-only); dismiss returns to settle/pay without auto-retry.
• Cancel is quiet (Twint-style); order stays unpaid; pay is locked for the whole collection window to avoid empty-settle races after a reported failure.

OpenSpec change: sumup-payment-fail-cancel-ux.

Test plan

• Unit: sumupCheckout cancel during poll terminates and rejects with cancelled
• Unit: timeout/failed status still terminate and do not return a payment
• Unit: resolvePayment sets failure message on fail, not on cancel; onSumupHide on both
• Unit: App overlay shows Abbrechen / failure sheet and wires cancel + dismiss
• Unit: useSplitPay ignores re-entry while collection is in flight
• Unit: PayOrderView mock includes terminalPaymentBusy lock exports
• Manual: start SumUp connected on register/table/collective → Abbrechen → still on settle, can pay again with another method
• Manual: decline/timeout → failure sheet → OK → retry or other method; no “nothing to settle”

Release

• No release (merge only; latest Pi images update)
• release:patch — bugfix release
• release:minor — feature release
• release:major — breaking release

Add exactly one release label to the PR if you want a semver bump, Git tag, and GitHub Release on merge. Do not edit VERSION manually — automation commits the bump on this branch after you add the label.

See docs/RELEASE.md.