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

[claude]

CLAUDE.md compliance: Comments should not be added inside private functions.

From CLAUDE.md: "NEVER add code comments to private functions, classes, etc"

These two new comments are inside private suspend fun proceedWithPayment():

bitkit-android/app/src/main/java/to/bitkit/viewmodels/AppViewModel.kt

Lines 1758 to 1764 in e16b3fa

	// When the invoice has a built-in amount, pass null so LDK uses the
	// invoice's native msat precision (avoids truncation to whole sats).
	val paymentAmount = if (decodedInvoice.amountSatoshis > 0uL) null else amount
	// For display/UI purposes, use the invoice amount (in sats) when available.
	val displayAmountSats = decodedInvoice.amountSatoshis.takeIf { it > 0uL } ?: amount ?: 0uL

Consider removing the inline comments — the logic is self-evident from the variable names (paymentAmount vs displayAmountSats).

Suggested change

	// For display/UI purposes, use the invoice amount (in sats) when available.
	val paymentAmount = if (decodedInvoice.amountSatoshis > 0uL) null else amount
	val displayAmountSats = decodedInvoice.amountSatoshis.takeIf { it > 0uL } ?: amount ?: 0uL

[claude]

Code review

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