fix: wait for the fee rate before swipe to pay

[ovitrif]

Closes #775

This PR keeps Swipe To Pay disabled until the fee rate has loaded.

Description

• Keeps Swipe To Pay disabled, with the swipe's loading spinner, while an on-chain payment from the wallet has no fee rate yet, so a swipe can no longer fail with "Fee rate not set" while the fee-rate fetch is still running.
• Resolves a missing fee rate at the start of the payment submit, before any warning, PIN check or Paykit payment proof, so a payment request is never consumed without a fee rate. This also covers the automatic payment path, which does not go through the swipe.
• Lightning payments and hardware wallet sends keep their current rules: neither waits on the wallet fee rate.
• Adds a changelog fragment.

Out of Scope

• Bitkit/Views/Wallets/Send/SendSheet.swift: the fee-rate fetch itself (its timeout, error logging and when it runs) is unchanged; if the fetch fails for good the swipe stays disabled until the funding source is switched or the sheet is reopened.
• Failed send leaving the payment request unpayable: tracked in bug: a failed send leaves the incoming paykit request stuck and unpayable #776 and fixed in fix: restore failed payment requests #826.
• bitkit-android: not affected. Its on-chain send resolves its own fee rate at send time with a cached fallback, so no twin issue is needed.

Design

N/A — no design available. The swipe reuses its existing disabled and loading states.

Preview

QA Notes

Journeys

N/A — not drivable; see Manual Tests.

Manual Tests

• Throttle the network so the Blocktank info request is slow → open a Paykit payment request or an on-chain invoice → Send confirmation from savings → the swipe is dimmed with a spinner and does not move; once the fee rate loads it becomes enabled and the payment goes through — network throttling not in Capabilities
• regression: Lightning invoice → Send confirmation → the swipe is enabled immediately, without waiting for the fee rate — network throttling not in Capabilities

Automated Checks

• added SendConfirmationSwipeTests.swift — on-chain payment without a fee rate disables the swipe, and it enables once a rate is set
• added SendConfirmationSwipeTests.swift — Lightning and hardware payments never wait on the wallet fee rate

[ovitrif]

Pushed 9be9583: renamed the changelog fragment to this PR's number, as pr.md asks once the PR exists. No code change.