feat: add boltz savings swap

[coreyphillips]

This PR adds a swap-based path for transferring spending balance to savings, so Lightning funds can move on-chain through a Boltz reverse swap instead of only by closing a channel. It ports the equivalent Android feature (synonymdev/bitkit-android#1081) to iOS, including its later refinements.

Description

• Adds a Boltz reverse swap flow to the transfer-to-savings journey. The confirm screen shows a live quote (amount to savings, network fee, service fee, and the amount you will receive) and lets the user proceed with a swap or fall back to closing the channel.
• The flow is gated behind a new "Enable Savings Swap" toggle under Dev Settings while its UI is being finalised, so nothing changes for users until it is switched on. Dev Settings also gets a Swaps section grouping that toggle with the existing swaps list.
• Swaps are offered on mainnet only. Boltz's testnet deployment is deprecated and regtest expects a local backend, so on every other network, and whenever the dev toggle is off, the journey behaves exactly as it did before this PR: the swipe closes the channel, with no quote, fees, slider, or extra close action, and the updates stream never starts.
• Closing a channel is the default and a priced quote is the only thing that upgrades a transfer to a swap. Without one, for any reason (unsupported network, Boltz unreachable, or an amount below the swap minimum), the swipe closes the channel rather than doing nothing. The quote fetch is bounded so a hanging Boltz request cannot leave the swipe stuck loading.
• Holds back a small reserve for Lightning routing fees so the hold invoice can actually be paid.
• Waits for the on-chain claim during the progress step. If the claim has not landed within the wait window (30s), the screen shows a "settling on-chain" state so the transfer reads as committed and completing in the background rather than as a false success.
• Resumes any pending swaps when the wallet starts and refreshes balances automatically once a swap lands on-chain. The updates stream is managed in the wallet view model: it only starts where swaps are supported and enabled, retries with backoff until it starts (a paid swap needs the stream to broadcast its claim), stops with the node, and restarts on the next wallet start. The progress screen also ensures the stream is running before starting a swap.
• Claims reverse swaps as soon as their lockup hits the mempool (acceptZeroConf) instead of waiting for a confirmation, for faster settlement.
• Adds a Swaps section under Dev Settings to list swaps and claim a reverse swap, backed by a new service that wraps the Boltz module. The manual claim is a recovery tool, so it is offered for any unclaimed reverse swap that has not reached a terminal state, including a swap still marked created locally. A stalled updates stream can leave the cached status behind while the funds are already claimable on-chain, and the chain is the source of truth.
• Adds a reusable continuous amount slider component used by the swap amount controls, and an optional loading state on the swipe-to-confirm button so it cannot be swiped while a quote is still being fetched.
• Bumps bitkit-core to 0.5.2, which adds the Boltz module this flow depends on and the acceptZeroConf option on the updates stream.

Linked Issues/Tasks

Android counterpart: synonymdev/bitkit-android#1081

Screenshot / Video

Insert relevant screenshot / recording

QA Notes

Manual Tests (swap path needs mainnet plus Dev Settings → Enable Savings Swap)

• 1. With a spending balance, Transfer to Savings → Savings Confirm: quote shows amount to savings, network fee, service fee, and you'll receive.
• 2. Savings Confirm → drag the amount slider: quote recomputes for the new amount.
• 3. Savings Confirm with a balance below the swap minimum: no quote or fees show and the swipe closes the channel instead, with the separate close action hidden.
• 4a. Savings Confirm → swipe to confirm → Savings Progress: completes after the on-chain claim, or shows the "settling on-chain" state if the claim is still landing.
  • 4b. After claim: savings balance reflects the swapped funds without a manual sync.
• 5. regression: Savings Confirm → Close channel instead: falls back to the channel-close path.
• 6. regression: Start the app with a pending swap: swap resumes and balance updates once it lands on-chain.
• 7a. Dev Settings → Swaps → tap a swap → Swap Details: Claim shows for any unclaimed reverse swap that is not in a terminal state, including one still marked created locally, the recovery case.
  • 7b. Swap Details of a claimed, expired, refunded, or failed swap, or any submarine swap: no Claim button.
• 8. regression: On a regtest or testnet build, Transfer to Savings → Savings Confirm: no quote, fees, slider, or close-instead action, and the swipe closes the channel as before.
• 9. regression: With Dev Settings → Enable Savings Swap off, Transfer to Savings → Savings Confirm: no quote, fees, slider, or close-instead action, and the swipe closes the channel as before.
• 10. Dev Settings → Swaps section: toggling Enable Savings Swap persists across an app restart.

Automated Checks

• Unit tests added: savings swap quote fee breakdown, claimable status gating with permissive recovery semantics, the swap gate (unsupported off mainnet, off without the dev flag, and the dev flag alone not enabling it off mainnet), and the transfer resolving to the channel-close path without a quote, in BitkitTests/SavingsSwapTests.swift.
• Ran locally against bitkit-core 0.5.2: simulator Debug build succeeds and SavingsSwapTests pass.
• No changelog fragment: the flow is behind a dev toggle, so it is not user-facing yet.

[greptile-apps]

Greptile Summary

This PR adds a Boltz reverse-swap path for moving Lightning funds to on-chain savings. The main changes are:

• Live swap quotes with network and service fee details.
• A bounded claim wait with a background settling state.
• Automatic swap resumption, claim handling, and balance refresh.
• Manual swap inspection and claim recovery in Dev Settings.
• A reusable continuous amount slider and swap-focused tests.
• An update to bitkit-core 0.5.2.

Confidence Score: 5/5

This looks safe to merge.

• The payment call returns after initiation, so it does not delay the bounded claim wait.
• Swap events are subscribed before payment starts.
• No blocking issue related to the settlement timeout remains in the updated code.

Important Files Changed

Filename	Overview
Bitkit/ViewModels/TransferViewModel.swift	Adds reverse-swap quotes, payment initiation, and a bounded wait for claim events.
Bitkit/ViewModels/WalletViewModel.swift	Adds swap-update startup, retry, shutdown, and balance refresh handling.
Bitkit/Services/BoltzService.swift	Wraps the Boltz API and forwards swap lifecycle events through asynchronous streams.
Bitkit/Views/Transfer/SavingsConfirmView.swift	Adds swap quotes, amount selection, and the channel-close fallback.
Bitkit/Views/Transfer/SavingsProgressView.swift	Runs the selected transfer path and shows pending claims as settling on-chain.
BitkitTests/SavingsSwapTests.swift	Covers quote calculations and manual claim eligibility.

_{Reviews (2): Last reviewed commit: "refactor: simplify savings swap payment ..." | Re-trigger Greptile}