fix: use msat-precision invoices for fixed-amount LNURL withdraw

For LNURL-withdraw with sub-sat precision (e.g. 222538 msat), neither
floor (222 sats) nor ceiling (223 sats) matches the server's exact
amount range. Add receiveMsats/createInvoiceMsats to create invoices
with native msat precision, used for fixed-amount LNURL withdrawals.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ben-kaufman

app/src/main/java/to/bitkit/repositories/LightningRepo.kt

@@ -897,6 +897,15 @@ class LightningRepo @Inject constructor(
         runCatching { lightningService.receive(amountSats, description, expirySeconds) }
     }
 
+    suspend fun createInvoiceMsats(
+        amountMsats: ULong,
+        description: String,
+        expirySeconds: UInt = 86_400u,
+    ): Result<String> = executeWhenNodeRunning("createInvoiceMsats") {
+        updateGeoBlockState()
+        runCatching { lightningService.receiveMsats(amountMsats, description, expirySeconds) }
+    }
+
     @Suppress("ForbiddenComment")
     suspend fun fetchLnurlInvoice(
         callbackUrl: String,

app/src/main/java/to/bitkit/services/LightningService.kt

@@ -590,15 +590,19 @@ class LightningService @Inject constructor(
     }
 
     suspend fun receive(sat: ULong? = null, description: String, expirySecs: UInt = 3600u): String {
+        return receiveMsats(amountMsat = sat?.let { it * 1000u }, description = description, expirySecs = expirySecs)
+    }
+
+    suspend fun receiveMsats(amountMsat: ULong? = null, description: String, expirySecs: UInt = 3600u): String {
         val node = this.node ?: throw ServiceError.NodeNotSetup()
 
         val message = description
 
         return ServiceQueue.LDK.background {
-            val bolt11Invoice: Bolt11Invoice = if (sat != null) {
+            val bolt11Invoice: Bolt11Invoice = if (amountMsat != null) {
                 node.bolt11Payment()
                     .receive(
-                        amountMsat = sat * 1000u,
+                        amountMsat = amountMsat,
                         description = Bolt11InvoiceDescription.Direct(description = message),
                         expirySecs = expirySecs,
                     )

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

@@ -1887,23 +1887,23 @@ class AppViewModel @Inject constructor(
                 return@launch
             }
 
-            val withdrawAmountSats = if (lnurl.data.isFixedAmount()) {
-                lnurl.data.fixedWithdrawAmountSat()
+            val invoice = if (lnurl.data.isFixedAmount()) {
+                lightningRepo.createInvoiceMsats(
+                    amountMsats = lnurl.data.maxWithdrawable,
+                    description = lnurl.data.defaultDescription,
+                    expirySeconds = 3600u,
+                )
             } else {
-                _sendUiState.value.amount.coerceAtLeast(
+                val withdrawAmountSats = _sendUiState.value.amount.coerceAtLeast(
                     (lnurl.data.minWithdrawable ?: 0u) / 1000u
                 )
-            }
-
-            _sendUiState.update {
-                it.copy(amount = withdrawAmountSats)
-            }
-
-            val invoice = lightningRepo.createInvoice(
-                amountSats = withdrawAmountSats,
-                description = lnurl.data.defaultDescription,
-                expirySeconds = 3600u,
-            ).getOrNull()
+                _sendUiState.update { it.copy(amount = withdrawAmountSats) }
+                lightningRepo.createInvoice(
+                    amountSats = withdrawAmountSats,
+                    description = lnurl.data.defaultDescription,
+                    expirySeconds = 3600u,
+                )
+            }.getOrNull()
 
             if (invoice == null) {
                 setSendEffect(SendEffect.NavigateToWithdrawError)