Write the Client Integration Guide for Accepting Payments

[codebestia]

Write the guide for an application developer building the payer-facing side: constructing an invoice, presenting it, submitting the payment transaction, handling authorization, and confirming settlement.

Proposed steps:

• Create docs/guides/accepting-payments.md.
• Document the integration flow end to end: create invoice server-side → present amount and token to the payer → payer approves and signs → submit pay_invoice → confirm via get_invoice and events.
• Document authorization concretely: what require_auth requires from the payer's transaction, and how token transfer authorization is included.
• Document handling the fiat-priced case: quoting with resolve_invoice_amount, re-quoting before submission, and communicating slippage to the user.
• Document partial payments and how a client should present a partially paid invoice.
• Document error handling: mapping the common contract errors to user-facing messages.
• Include client code samples for building and submitting the transaction.

Acceptance criteria:

• docs/guides/accepting-payments.md exists.
• The end-to-end flow is documented with working client code samples.
• Authorization requirements for the payer are stated precisely.
• Fiat quoting/re-quoting and partial-payment presentation are covered.
• Common errors are mapped to user-facing handling.