refactor(send): move route fee estimation out of the SwiftUI view layer

[QuantumExplorer]

Issue being fixed or feature implemented

CodeRabbit flagged on #1057 that SendConfirmSheet.networkFeeCredits calls PlatformWalletManager.estimateShieldedFee (FFI) and performs per-route fee calculations inside a SwiftUI View — a pattern the repo's SwiftUI-first guardrails ban from View structs. The pattern predates #1057 (that PR only added the .platformToShielded case following the existing convention), so the refactor was declined there as out of scope and split into this PR.

What was done?

• SendViewModel now owns both confirm-sheet computations:
  • confirmNetworkFeeCredits — the per-route fee switch. The pool-spending routes reuse the existing shieldedFeeKind(for:) mapping instead of duplicated estimateShieldedFee calls, .platformToPlatform reuses the existing platformTransferFeeReserveCredits constant instead of a bare 100_000_000 literal, .platformToCore reads withdrawalPreflight?.estimatedFee directly, and .platformToShielded keeps feat(wallet): pay external shielded addresses from the Platform balance #1057's flat 2-action estimate.
  • confirmTotalDuffs — the Total row's Core → Shielded lock-value derivation (CoreToShieldedAmountPolicy is FFI-backed too, so leaving it would have kept an FFI call in the view).
• SendConfirmSheet receives the two values as stored let properties and only formats them. SendScreen.swift is now free of PlatformWalletManager / CoreToShieldedAmountPolicy references.
• Behavior is identical route by route: same estimators, same values, same — fallbacks. Update timing is unchanged — the sheet's parameters were already re-evaluated through the observed view model (the same mechanism as the existing withdrawalFeeCredits parameter).

How Has This Been Tested?

Clean dashpay scheme simulator build (-sdk iphonesimulator, arm64) on top of current develop (includes #1057). Pure code motion with no rendering change, verified by diff review of each route's estimator against the previous switch. The unit-test target is broken pre-existing on this branch lineage, so no new tests were added.

Breaking Changes

None.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 53bb20d and 2722b29.

📒 Files selected for processing (2)

• DashWallet/Sources/UI/Payments/Pay/SendScreen.swift
• DashWallet/Sources/UI/Payments/Pay/SendViewModel.swift

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[thepastaclaw]

🕓 Ready for review — 8 ahead in queue (commit 2722b29)
Queue position: 9/42 · 2 reviews active
ETA: start ~21:25 UTC · complete ~22:25 UTC (median 1h across 30 recent reviews; 2 slots)
Queued 6h 41m ago · Last checked: 2026-08-31 17:20 UTC