fix: remove dead fixedWithdrawAmountSat and simplify conditional

fixedWithdrawAmountSat() had no production call sites after switching
to createInvoiceMsats for fixed-amount withdrawals and satsCeil for
display. Remove the function, its test, and simplify the redundant
displayAmount conditional.

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

Author: ben-kaufman

app/src/main/java/to/bitkit/ext/Lnurl.kt

@@ -57,10 +57,3 @@ fun LnurlWithdrawData.isFixedAmount(): Boolean {
     val min = minWithdrawable ?: 0u
     return min == maxWithdrawable || (min > 0u && minWithdrawableSat() > maxWithdrawableSat())
 }
-
-/**
- * The amount in whole sats to use when creating a withdraw invoice for a fixed-amount request.
- *
- * Uses floor division so the invoice amount never exceeds `maxWithdrawable` in msats.
- */
-fun LnurlWithdrawData.fixedWithdrawAmountSat(): ULong = maxWithdrawable / MSATS_PER_SAT

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

@@ -1527,7 +1527,7 @@ class AppViewModel @Inject constructor(
             return
         }
 
-        val displayAmount = if (isFixed) data.minWithdrawableSat() else minWithdrawable
+        val displayAmount = minWithdrawable
 
         _sendUiState.update {
             it.copy(

app/src/test/java/to/bitkit/ext/LnurlExtTest.kt

@@ -119,11 +119,6 @@ class LnurlExtTest : BaseUnitTest() {
         assertEquals(false, data.isFixedAmount())
     }
 
-    @Test
-    fun `fixedWithdrawAmountSat floors to avoid exceeding max`() {
-        val data = withdrawData(minWithdrawable = 500_500u, maxWithdrawable = 500_500u)
-        assertEquals(500u, data.fixedWithdrawAmountSat())
-    }
 
     private fun lnurlPayData(
         minSendable: ULong = 1_000u,