Keep "currency to Sats" empty when the BTC rate is, and whole-Sats only

Clearing the Manual rate cleared pricedCurrencyCodes but left
rateDisplays stale, so "currency to Sats" kept showing a value
computed from the old rate even after "BTC to currency" went empty.
refresh()'s early-return path now clears rateDisplays too.

oneCurrencyToSats() also now derives from defaultCurrencyRate()
directly instead of re-reading rateDisplays independently, so the
two rates structurally can't disagree about whether a rate is known,
and formats to a whole number of Sats (0 decimals) instead of 2 —
fractional Sats aren't a meaningful unit to show.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/ui/ConverterUiStateDisplay.kt

@@ -26,10 +26,14 @@ fun ConverterUiState.exceedsMaxSupply(): Boolean {
 fun ConverterUiState.defaultCurrencyRate(): String =
     rateDisplays[defaultCurrencyCode]?.takeIf { it.isNotEmpty() } ?: ""
 
-/** The current "1 [ConverterUiState.defaultCurrencyCode] = ? Sats" rate, as a plain number. */
+/**
+ * The current "1 [ConverterUiState.defaultCurrencyCode] = ? Sats" rate, as a whole (never
+ * fractional) number of Sats — empty whenever [defaultCurrencyRate] is, so the two rates never
+ * disagree about whether a rate is currently known.
+ */
 fun ConverterUiState.oneCurrencyToSats(): String {
-    val rate = rateDisplays[defaultCurrencyCode]?.toBigDecimalOrNull() ?: return ""
-    return CurrencyConverter.satsPerCurrencyUnit(rate)?.let { formatAmount(it, 2) } ?: ""
+    val rate = defaultCurrencyRate().toBigDecimalOrNull() ?: return ""
+    return CurrencyConverter.satsPerCurrencyUnit(rate)?.let { formatAmount(it, 0) } ?: ""
 }
 
 /** The pinned, non-removable "Current Currency" shown in the currency picker's own section. */

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/ui/PriceViewModel.kt

@@ -143,10 +143,16 @@ class PriceViewModel(
     fun refresh() {
         if (currentSource === manualSource && manualSource.rate == null) {
             // No rate typed in (yet): nothing to price fiat currencies with, so clear any stale
-            // rate this might otherwise still compute fiat amounts from.
+            // rate this might otherwise still compute fiat amounts (or the derived "currency to
+            // Sats" rate) from.
             rates = null
             _uiState.update {
-                it.copy(isLoading = false, errorMessage = null, pricedCurrencyCodes = emptySet())
+                it.copy(
+                    isLoading = false,
+                    errorMessage = null,
+                    pricedCurrencyCodes = emptySet(),
+                    rateDisplays = emptyMap(),
+                )
             }
             return
         }

shared/src/commonTest/kotlin/xyz/tyiu/satsprice/ui/ConverterUiStateDisplayTest.kt

@@ -0,0 +1,36 @@
+package xyz.tyiu.satsprice.ui
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class ConverterUiStateDisplayTest {
+
+    @Test
+    fun oneCurrencyToSats_isInverseOfTheBtcRate() {
+        val state = ConverterUiState(defaultCurrencyCode = "USD", rateDisplays = mapOf("USD" to "50000.00"))
+
+        // 1 BTC = 50,000 USD, so 1 USD is worth 100,000,000 / 50,000 = 2,000 Sats.
+        assertEquals("2000", state.oneCurrencyToSats())
+    }
+
+    @Test
+    fun oneCurrencyToSats_roundsToAWholeNumberOfSats() {
+        // 1 BTC = 78,293.19 USD, so 1 USD is worth ~1,277.24 Sats — never shown fractionally.
+        val state = ConverterUiState(defaultCurrencyCode = "USD", rateDisplays = mapOf("USD" to "78293.19"))
+
+        assertEquals("1277", state.oneCurrencyToSats())
+    }
+
+    @Test
+    fun oneCurrencyToSats_isEmptyWheneverDefaultCurrencyRateIs() {
+        // No rate for the default currency at all (e.g. Manual with nothing typed in yet).
+        val noRate = ConverterUiState(defaultCurrencyCode = "USD", rateDisplays = emptyMap())
+        assertEquals("", noRate.defaultCurrencyRate())
+        assertEquals("", noRate.oneCurrencyToSats())
+
+        // A rate is known, just not for the default currency.
+        val otherCurrencyOnly = ConverterUiState(defaultCurrencyCode = "USD", rateDisplays = mapOf("EUR" to "45000"))
+        assertEquals("", otherCurrencyOnly.defaultCurrencyRate())
+        assertEquals("", otherCurrencyOnly.oneCurrencyToSats())
+    }
+}