fix: explain Monero payment ID destination conflict

[sethforprivacy]

Issue Number (if Applicable): Fixes CW-996

Description

Rejects a multi-destination Monero transaction when any non-empty destination contains an integrated payment ID, while preserving valid single-recipient and ordinary multi-recipient sends. The typed failure is mapped to localized error copy across all supported ARB locales.

Validation

• Focused review: no blocking findings

[sethforprivacy]

Smoke test result: BLOCKED (environmental) — no PR defect found

On-device attempt against the exact PR CI APK (Android emulator, Android 14 / API 34) using Trailblaze. The PR's rejection path could not be demonstrated end-to-end via the UI in this environment; the gate logic itself was verified in source at the exact head. No PR defect was found.

Tested artifact

• APK: GitHub Actions run 31333364389 ("Cake Wallet Android", success), artifact "android apk" id 9044834158
• Head SHA: d5c4c6f66aac27afd5b70d5dfecda54a6ced8025
• Canonical APK: app-arm64-v8a-release.apk, SHA-256 d556f429c13259af1c782fc7e533f6e48a02f2c71841dc3995c3ac0d497afaf6 (SHA-1 matches CI sidecar)
• Installed package: com.cakewallet.test_fixcw996moner v6.4.0 (6419)

What was verified

• Wallet setup, node, and sync fully working: fresh Monero wallet synced against a real daemon (guest→host bridge to nodes.hashvault.pro:18081, live height ~3,737,063 advancing; logcat onNewBlock), send form reached with multi-recipient capability ("Add another receiver"), two recipients enterable (standard + integrated addresses, both format-valid checksum-verified mainnet).
• Case A (CORE: standard + integrated, 0.01 each): confirmation sheet appears, then Transaction Error: You do not have enough XMR to send this amount. The PR's payment-ID rejection is NOT reached.
  Root cause (source-verified at exact head): a pre-existing balance gate in cw_monero/lib/monero_wallet.dart (~line 458) throws before transaction_history.dart createTransactionMultDest (~line 470) runs, and CW-996's payment-ID check sits at the top of createTransactionMultDest. With a 0-balance wallet the balance gate always fires first, making the payment-ID rejection unreachable in-app.
• Case B (negative control, two standard addresses): identical "not enough XMR" result — pair indistinguishable at 0 balance, as the payment-ID gate was not reached in either case.
• Case C (single integrated recipient): single integrated send is accepted through validation (confirmation sheet parses the integrated address), no payment-ID rejection — consistent with the PR's "single integrated allowed" contract.
• Code path at exact head: transaction_history.dart:250-255 throws MoneroMultipleDestinationsPaymentIdException before the native call; send_view_model.dart maps it to error_text_payment_id_multiple_recipients; the ARB string ("Transactions cannot contain more than one destination when paying to a payment ID.") is present. The balance gate is pre-existing (present in parent commit too), not introduced by this PR.

Blocker to full UI demonstration: demonstrating the UI rejection requires a wallet balance ≥ the two recipient amounts (unlocked XMR). No funding path was available in this environment, so the end-to-end UI rejection could not be exercised. PR logic is present and correct per source inspection at the exact head; availability check isIntegrated / drizzle gate behavior was verified via the gateway logic and the single-recipient control.

Evidence

• .trailblaze/pr-smokes/pr-3501/evidence/ (+ retry2-sync-enabled/): case screenshots (2 recipients + balance error, 2-standard control, single-integrated), node list, seed screen, EVIDENCE.md, wallet notes, trails.