Make MAX and multi-recipient send exclusive

[j0ntz]

CHANGELOG

Does this branch warrant an entry to the CHANGELOG?

• Yes
• No

Dependencies

none

Requirements

If you have made any visual changes to the GUI. Make sure you have:

• Tested on iOS device
• Tested on Android device
• Tested on small-screen device (iPod Touch)
• Tested on large-screen device (tablet)

Description

Asana: https://app.asana.com/0/1215088146871429/1209203315991847

MAX spends the entire spendable balance, which only has a coherent meaning for a
single recipient. Since the send scene gained 1-to-many support, MAX and
multi-send conflict in both orderings:

• Tap MAX on recipient 2 and nothing happens. The recalculation effect only ever
  honors maxSpendSetter === 0, so a max requested by any later target sets
  state that nothing consumes.
• Tap MAX on recipient 1, then add a second recipient with any amount, and the
  send fails with Insufficient Funds, because recipient 1 already claims the
  whole balance.

The operator's decision on the task (after twice rejecting a per-target
remainder calculation) is to make the two mutually exclusive. This does that in
both directions:

1. handleFlipInputModal passes hideMaxButton to FlipInputModal2 once the
   send has more than one spend target. The prop already existed and is gated in
   ExchangedFlipInput2 alongside noMaxSpend, so no new plumbing.
2. renderAddAddress withholds the "Add Another Address" row while a max spend
   is set. Clearing the max instead would silently discard an amount the user
   deliberately chose, so the entry point is withheld rather than the amount
   rewritten.

Single-recipient sends are unchanged, and currencies with maxSpendTargets < 2
never reach either branch.

Testing

Driven on the iOS simulator against a funded Bitcoin Testnet wallet
(maxSpendTargets 32, same multi-send path as mainnet BTC):

• One recipient: MAX is present, and tapping it fills the amount as before.
• With MAX applied: "Add Another Address" is gone.
• Retyping the amount by hand on that same screen (which clears the max) brings
  the row back, so the two frames differ only in maxSpendSetter.
• Two recipients: the amount modal has no MAX button.
• A 2-recipient send still broadcasts to Transaction Success.

tsc --noEmit, jest (SendScene2 snapshots unchanged), and verify-repo.sh all pass.

[j0ntz]

📸 Test evidence

max present single recipient

max set add address hidden

manual amount add address visible

max hidden second recipient

multisend broadcast success

Captured by the agent's in-app test run (build-and-test).

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