Hide refresh for Manual, seed its rate from another source

Refreshing has no meaning for a manually-typed rate, so the button
(and its loading spinner) is now hidden while Manual is selected.

Switching to Manual for the first time now seeds its rate field from
whichever source was active before, falling back to any other
source's last cached rate for the default currency. When neither is
available, the rate and amount fields are left empty instead of
showing stale values.

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

iosApp/iosApp/ContentView.swift

@@ -65,15 +65,17 @@ struct ContentView: View {
                     } else {
                         Spacer()
                     }
-                    if state.isLoading {
-                        ProgressView()
-                    } else {
-                        Button {
-                            viewModel.refresh()
-                        } label: {
-                            Image(systemName: "arrow.clockwise")
+                    if !state.isManualSource {
+                        if state.isLoading {
+                            ProgressView()
+                        } else {
+                            Button {
+                                viewModel.refresh()
+                            } label: {
+                                Image(systemName: "arrow.clockwise")
+                            }
+                            .buttonStyle(.borderless)
                         }
-                        .buttonStyle(.borderless)
                     }
                 }
             }