Hide the Manual status line, clear fiat amounts when its rate is gone

"via Manual" had nothing useful left to say once the "updated" part
was dropped, so statusLine() now returns empty for Manual and both
UIs skip rendering it entirely instead of showing a blank line/footer.

onManualRateChanged also now clears manualSource.rate whenever the
field doesn't parse (emptied, or mid-edit on an incomplete number)
rather than only ever updating it on a successful parse. refresh()'s
early-return for a null Manual rate now also clears the cached rates
and pricedCurrencyCodes, so a cleared/incomplete manual rate stops
fiat amounts from continuing to show a stale computed value.

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

iosApp/iosApp/ContentView.swift

@@ -36,7 +36,11 @@ struct ContentView: View {
                     resource: MR.strings.shared.btc_to_currency,
                     args: [state.defaultCurrencyCode]
                 )),
-                footer: Text(state.statusLine)
+                footer: Group {
+                    if !state.statusLine.isEmpty {
+                        Text(state.statusLine)
+                    }
+                }
             ) {
                 Picker(
                     IosLocalizationKt.localizedString(resource: MR.strings.shared.price_source),