fix: cap onchain send amount to balance

[piotr-iohk]

Fixes #611

This PR caps the on-chain send amount to the available balance, disabling the Continue button when the entered amount exceeds spendable sats.

Description

The validateAmount function in AppViewModel only checked that the on-chain amount was above dust limit — it never compared against maxSendOnchainSats. This allowed users to proceed with amounts exceeding their balance. Lightning already had this check in place.

The fix adds amount <= maxSendOnchainSats to the on-chain validation branch, matching the Lightning behavior.

Preview

Screen.Recording.2026-03-26.at.15.31.44.mov

QA Notes

1. Fund an on-chain wallet with a known balance
2. Initiate an on-chain send (scan a BTC address QR)
3. Enter an amount greater than your available balance
4. Verify the Continue button is disabled
5. Enter an amount within balance — verify the Continue button is enabled
6. Verify Lightning send behavior is unchanged (should still cap correctly)

E2E test coverage: synonymdev/bitkit-e2e-tests#137

Made with Cursor

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

tAck

LGTM

Note that in order to get this to the level of polishing we had in the RN app, we should also prevent entering a higher amount than the max while using the numpad.

IIRC RN bitkit also had a toast or just red numbers while typing 🤷🏻