Add a "1 <currency> to Sats" field

Shows the inverse of the existing "1 BTC to <currency>" rate — how
many Sats one unit of the default currency is worth — right below it.
Useful since many currencies are worth less than, or a low multiple
of, a single Sat.

CurrencyConverter.satsPerCurrencyUnit(rate) does the actual division;
ConverterUiState.oneCurrencyToSats() derives the display string from
the already-fetched rate the same way defaultCurrencyRate() does, so
no new ViewModel state was needed. Wired into both Compose and the
iOS bridge/SwiftUI.

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

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/domain/CurrencyConverter.kt

@@ -44,4 +44,8 @@ object CurrencyConverter {
 
     fun fiatToSats(fiat: BigDecimal, rates: ExchangeRates, currency: String): BigDecimal? =
         fiatToBtc(fiat, rates, currency)?.let { btc -> btcToSats(btc) }
+
+    /** Sats worth 1 unit of a currency whose "1 BTC = ?" rate is [rate], or null if [rate] is zero. */
+    fun satsPerCurrencyUnit(rate: BigDecimal): BigDecimal? =
+        if (rate.isZero()) null else SATS_PER_BTC.divide(rate, DIVISION_MODE)
 }

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

@@ -4,6 +4,7 @@ import kotlinx.datetime.TimeZone
 import kotlinx.datetime.toLocalDateTime
 import xyz.tyiu.satsprice.CurrencyInfo
 import xyz.tyiu.satsprice.domain.CurrencyConverter
+import xyz.tyiu.satsprice.domain.formatAmount
 import xyz.tyiu.satsprice.domain.toBigDecimalOrNull
 import kotlin.time.Instant
 
@@ -25,6 +26,12 @@ fun ConverterUiState.exceedsMaxSupply(): Boolean {
 fun ConverterUiState.defaultCurrencyRate(): String =
     rateDisplays[defaultCurrencyCode]?.takeIf { it.isNotEmpty() } ?: ""
 
+/** The current "1 [ConverterUiState.defaultCurrencyCode] = ? Sats" rate, as a plain number. */
+fun ConverterUiState.oneCurrencyToSats(): String {
+    val rate = rateDisplays[defaultCurrencyCode]?.toBigDecimalOrNull() ?: return ""
+    return CurrencyConverter.satsPerCurrencyUnit(rate)?.let { formatAmount(it, 2) } ?: ""
+}
+
 /** The pinned, non-removable "Current Currency" shown in the currency picker's own section. */
 fun ConverterUiState.currentCurrency(): CurrencyInfo =
     availableFiatCurrencies.find { it.code == defaultCurrencyCode } ?: CurrencyInfo(defaultCurrencyCode, defaultCurrencyCode)

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

@@ -213,6 +213,18 @@ fun PriceScreen(
                             }
                         }
                     }
+
+                    val oneCurrencyToSats = state.oneCurrencyToSats()
+                    if (oneCurrencyToSats.isNotEmpty()) {
+                        Text(
+                            text = stringResource(MR.strings.currency_to_sats, state.defaultCurrencyCode),
+                            style = MaterialTheme.typography.bodyMedium,
+                        )
+                        Text(
+                            text = groupDigits(oneCurrencyToSats),
+                            style = MaterialTheme.typography.headlineSmall,
+                        )
+                    }
                 }
             }