feat(wallet): Max fills the fee-aware maxSendable

[romchornyi]

Issue being fixed or feature implemented

The "Max" buttons in the Send and Internal-transfer flows filled the raw balance, leaving no room for the transaction fee — and for the total-based ones, even including unconfirmed/immature funds that can't be spent. The resulting amount was not actually sendable, so the send failed to build or showed insufficient funds.

What was done?

Routed all three Max fills through WalletBalance.maxSendable (spendable minus the send fee reserve — the SwiftDashSDK replacement for DSAccount.maxOutputAmount that the rest of the app already caps Max at, e.g. ContactProfileSheet, BuyCreditsViewController, DashSpendPayViewModel, CrowdNode, and the WalletSendService "callers cap input at maxSendable" contract):

• SendViewModel.fillMaxFromWallet (Core legs) — was balance.total
• SendAmountModel.selectAllFundsWithoutAuth (classic amount screen) — was balance.spendable
• InternalTransferViewModel.fillMaxFromWallet (Core legs) — was balance.total

Only the Max fills changed; balance-card display and affordability checks that legitimately use total are untouched.

How Has This Been Tested?

Clean dashpay build (xcodebuild ... -scheme dashpay -sdk iphonesimulator ARCHS=arm64 build). Not yet smoke-tested on testnet — worth confirming that tapping Max then Send actually broadcasts for a Transparent send and a Core→Shielded internal transfer.

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

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