fix: polish trezor send loading states

[ben-kaufman]

Fixes #696

Description

This PR polishes the transient loading states in the Trezor send flow:

• shows blue hardware-wallet loading feedback while the selected source refreshes or a transaction preview is prepared
• prevents repeated source, Continue, fee, and swipe actions while their required hardware-wallet state is unavailable
• keeps the previous fee visible while a replacement preview loads, with the spinner on the speed icon
• separates a temporarily disabled swipe from the post-swipe submission spinner, matching the intended Figma behavior

This is stacked on #688. Review this PR against feat/trezor-send; it contains only the loading-state polish for that flow.

Linked Issues/Tasks

• Fixes Trezor send UI polish #696
• Depends on feat: add trezor send #688
• Related Android implementation: fix: send source & fee progress ui/ux bitkit-android#1195

Screenshot / Video

N/A — the resting screens are unchanged; this PR refines the transient loading and disabled states described in the QA steps below.

QA Notes

Manual Tests

• 1. Send → Amount → switch the funding source to Trezor: the source chip shows a blue spinner, and the source control and Continue cannot be triggered again until the available amount finishes refreshing.
• 2. Send → Confirm → switch to Trezor or change the fee speed: the previous fee stays visible, the speed icon shows a blue spinner, and the confirmation swipe remains disabled until a valid preview is ready.
• 3. Send → Confirm → swipe after the fee is ready: the knob reaches the right side and shows the submission spinner while the flow proceeds to Sign With Device.
• 4. regression: switch back to Spending or Savings: the normal source, fee, Continue, and swipe behavior remains available.

Automated Checks

• HwFundingSignerTests: 25 tests passed, including source-refresh, unavailable-fee, and preview-loading coverage.
• SwiftFormat lint passed for the changed files.
• git diff --check passed.

[greptile-apps]

Greptile Summary

This PR separates disabled and submitting states in the swipe control and adds explicit hardware-wallet funding-source and preview loading state across the send flow.

• Adds tintable activity indicators and loading support to shared controls.
• Tracks hardware funding-source and transaction-preview preparation in HwSendCoordinator.
• Disables repeated source, fee, Continue, and swipe actions while required hardware state is unavailable.
• Adds coordinator tests for source refresh and preview preparation.

Confidence Score: 4/5

The hardware-source loading failure path should be fixed before merging because a fee-estimation failure can trap the send screen with no usable source-selection or Continue action.

Hardware selection now starts an externally visible loading state that is cleared only after a refresh or preview, but the amount view can return before starting either operation when its fee rate is unavailable.

Files Needing Attention: Bitkit/Views/Wallets/Send/SendAmountView.swift; Bitkit/ViewModels/HwFundingSigner.swift

Important Files Changed

Filename	Overview
Bitkit/ViewModels/HwFundingSigner.swift	Adds request-scoped funding-source and preview loading flags, including stale-request invalidation and cancellation cleanup.
Bitkit/Views/Wallets/Send/SendAmountView.swift	Connects hardware loading to source and Continue controls, but can leave both unavailable when fee resolution fails before balance refresh.
Bitkit/Views/Wallets/Send/SendConfirmationView.swift	Presents hardware preview loading on the fee icon and disables source, fee, and swipe actions until preview readiness.
Bitkit/Components/SwipeButton.swift	Separates temporarily disabled presentation from the post-swipe spinner while retaining unified interaction blocking.
BitkitTests/HwFundingSignerTests.swift	Covers successful funding-source refresh and preview-loading transitions but not unavailable fee-rate or refresh-failure recovery.

Sequence Diagram

sequenceDiagram
    participant U as User
    participant V as SendAmountView
    participant C as HwSendCoordinator
    participant F as Fee estimation
    U->>V: Select Trezor
    V->>C: selectWallet(showsLoading: true)
    C-->>V: "isFundingSourceLoading = true"
    V->>F: Resolve selected fee rate
    alt Fee rate available
        V->>C: refreshAvailable(...)
        C-->>V: "Balance and loading=false"
    else Fee estimation fails
        V-->>V: Refresh guard returns
        Note over V,C: Loading remains true and source control stays disabled
    end

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