refactor: use the SDK payment link pay flow contract

[TaprootFreak]

Consumer side of step 3 of DFXswiss/packages#198: use the payment link pay-flow contract from
@dfx.swiss/react and drop the local copies.

Depends on DFXswiss/packages#201.

Changes

src/dto/payment-link.dto.ts keeps only what is genuinely local — WalletInfo, MetaMaskInfo, the
UI-only NoPaymentLinkPaymentStatus / ExtendedPaymentLinkStatus, and the LNURL wait shape. Everything
that describes the wire now comes from the package, under the package's names:

here, before	package
TransferInfo	TransferAmount
Amount	PaymentAmount
RecipientInfo	PaymentLinkRecipient
Quote	PaymentQuote

What the corrected types surfaced

The local copy had PaymentLinkPayRequest extends PaymentLinkPayTerminal, so error fields and quote
fields lived on one type and every access typechecked. The package models them as the two shapes the
API actually returns, which made four real assumptions visible:

• fetchPayRequest declared its response as a pay request and then read statusCode / message off
  it — the error case it was handling could not occur under its own type. It now reads
  PaymentLinkPayResponse and narrows with hasPaymentQuote.
• The assign screen and the "not assigned" branch read statusCode without narrowing; same fix.
• route and currency are optional on a pay request. The public-payment form passed route into
  URLSearchParams as a string and interpolated currency into a label — both assumed a value the
  API does not promise. route is now only sent when present.
• The Tag detail row read payRequest.tag, which only exists on a quoted response.

C2BPaymentMethod.KUCOINPAY is KUCOIN_PAY in the package.

Before this can go green

The package pin in package.json still points at the published @dfx.swiss/react. CI stays red
until DFXswiss/packages#201 is merged and published and the pin is raised here. Verified locally
against a build of that branch: tsc clean, npm run lint clean, full suite at 673 passing with no
new failures (the two suites failing on develop today still fail, unchanged).

Overlap

#1223 touches the same two contexts (wait-endpoint polling). The textual conflict is routine, but
worth a look when the second of the two lands: any code it adds that reads statusCode, message,
tag, route or currency off a pay request needs the same narrowing this PR introduces —
the compiler will say so, but only once both are in.

[github-actions]

🤖 PR Review Bot

❌ TypeScript: 37 errors

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