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

iosApp/iosApp/ContentView.swift

@@ -105,6 +105,17 @@ struct ContentView: View {
                         }
                     }
                 }
+
+                if !state.oneCurrencyToSats.isEmpty {
+                    HStack {
+                        Text(IosLocalizationKt.localizedFormattedString(
+                            resource: MR.strings.shared.currency_to_sats,
+                            args: [state.defaultCurrencyCode]
+                        ))
+                        Spacer()
+                        Text(NumberFormatKt.groupDigits(value: state.oneCurrencyToSats))
+                    }
+                }
             }
 
             if let error = state.errorMessage {