Keep the "currency to Sats" field visible, make it editable in Manual

Always render the field now (label included) with just its value
empty when unknown, rather than hiding the whole field.

In Manual source mode it's now a genuine input: typing into it
recomputes the "BTC to currency" rate the same way typing into that
field recomputes this one. Both edit the same underlying
ManualExchangeRateSource.rate via the same self-inverse division
(CurrencyConverter.satsPerCurrencyUnit of a rate gives Sats, and of a
Sats amount gives a rate back), tracked as a new manualSatsPerCurrencyInput
field alongside the existing manualRateInput.

Wired through PriceViewModel, the iOS bridge/ConverterViewModel, and
both UIs.

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

iosApp/iosApp/ContentView.swift

@@ -105,13 +105,22 @@ struct ContentView: View {
                     }
                 }
 
-                if !state.oneCurrencyToSats.isEmpty {
-                    HStack {
-                        Text(IosLocalizationKt.localizedFormattedString(
-                            resource: MR.strings.shared.currency_to_sats,
-                            args: [state.defaultCurrencyCode]
-                        ))
-                        Spacer()
+                HStack {
+                    Text(IosLocalizationKt.localizedFormattedString(
+                        resource: MR.strings.shared.currency_to_sats,
+                        args: [state.defaultCurrencyCode]
+                    ))
+                    Spacer()
+                    if state.isManualSource {
+                        NumericField(
+                            placeholder: "",
+                            value: state.manualSatsPerCurrencyInput,
+                            keyboardType: .decimalPad,
+                            sanitize: sanitizeDecimalInput,
+                            onChange: { viewModel.onManualSatsPerCurrencyChanged($0) },
+                            alignment: .trailing
+                        )
+                    } else if !state.oneCurrencyToSats.isEmpty {
                         Text(NumberFormatKt.groupDigits(value: state.oneCurrencyToSats))
                     }
                 }

iosApp/iosApp/ConverterViewModel.swift

@@ -46,4 +46,8 @@ final class ConverterViewModel: ObservableObject {
     func onManualRateChanged(_ value: String) {
         bridge.onManualRateChanged(value: value)
     }
+
+    func onManualSatsPerCurrencyChanged(_ value: String) {
+        bridge.onManualSatsPerCurrencyChanged(value: value)
+    }
 }