fix: avoid msat truncation when paying invoices with built-in amounts

[ben-kaufman]

Summary

• Bump bitkit-core from 0.1.38 to v0.1.56 which rounds up sub-satoshi invoice amounts (ceiling division instead of floor)
• Stop overriding the amount for invoices that already have one — pass null so LDK uses the invoice's native msat precision instead of our truncated sats value converted back to msat
• Only pass the amount for zero-amount invoices where the user specifies it

Test plan

• E2E: pay invoices created with lnd.addInvoice({ valueMsat }) using amounts 222538, 222222, 500500 msat
• Verify zero-amount invoice flow still works
• Verify LNURL pay flow still works
• Verify quickpay flow still works

Depends on synonymdev/bitkit-core#85
Closes #877

🤖 Generated with Claude Code

[ovitrif]

utAck
Will test too

[piotr-iohk]

Observations from testing:

1) The fix rounds up to full sats precision for the payment and display amount, so ln invoice with 500500 msats becomes 501 sats payment. In bitkit RN it seems that the displayed number of sats for such invoice still remains 500 - seems that, perhaps, Bitkit RN behind the scenes falls back to msats precision for the payment and just displays the floor sats amount. Just want to make sure we want that behaviour and not follow the Bitkit RN.
See example, for the 500500 invoice for Bitkit RN (left) and Bitkit android native:
Invoice generated with:

$ ./bitcoin-cli getlninvoice --msat 500500

from synonymdev/bitkit-e2e-tests#141.

Screen.Recording.2026-04-02.at.12.58.54.mov

---

2) There is an issue with lnurl-pay and and lnurl-withdrawal.

For testing on local regtest, more utility commands for bitcoin-cli can be used, added in synonymdev/bitkit-e2e-tests#141.

  LNURL (requires LNURL server, default: http://localhost:30001):
    startlnurlserver [--port N]        Start LNURL server using current e2e Bitcoin/LND
    stoplnurlserver                    Stop LNURL server container
    checklnurl                         Diagnose LNURL health, routing, and adb reverse
    getlnurlpay --msat N               Create LNURL-pay with fixed msat amount
    getlnurlwithdraw --msat N          Create LNURL-withdraw with fixed msat amount

$ ./bitcoin-cli startlnurlserver
$ ./bitcoin-cli checklnurl
$ ./bitcoin-cli getlnurlpay --msat 500500 
$ ./bitcoin-cli getlnurlwithdraw --msat 500500 

⚠️ lnurl-pay with msat precision shows 0 amount and cannot be paid. (Works on bitkit RN). The video is for lnurl-pay with 500500 msats $ ./bitcoin-cli getlnurlpay --msat 500500 - Bitkit RN on the left.

Screen.Recording.2026-04-02.at.13.53.59.mov

⚠️ lnurl-withdrawal with msat precision produces error toast. Video for $ ./bitcoin-cli getlnurlwithdraw --msat 500500 (note that lnurl-withdrawal with msat precision also doesn't seem to work properly on Bitkit RN - no error toast there, but the request not working)

Screen.Recording.2026-04-02.at.14.26.12.mov

[claude]

Code review

1 issue found.

---

Missing CHANGELOG.md entry

This PR has a fix: prefix but does not include an update to CHANGELOG.md. Per CLAUDE.md:

ALWAYS add an entry under ## [Unreleased] in CHANGELOG.md for feat: and fix: PRs

Please add an entry under ## [Unreleased] → ### Fixed, for example:

### Fixed
- Avoid msat truncation when paying invoices with built-in amounts #879