chore: update paykit to rc56 #1366

Twin: synonymdev/bitkit-ios#824
Refs: #1365

Problem or use case

Bitkit pins Paykit 0.1.0-rc55. Paykit 0.1.0-rc56 adds optional conversion terms, payment deadlines, allowance and quote fields to payment requests and proofs; taking it now avoids a data migration later.

Proposed solution

Update to the published Paykit 0.1.0-rc56 and adapt the existing Bitcoin payment request and proof calls, without enabling new payment features.

Done when the existing contact payment and payment request flows work on rc56.

Additional context

rc56 still uses Pubky 0.11; the Pubky 0.12 upgrade stays tracked separately.