feat(apple): collect transfers held for this device

Adds the opt-in foreground check and an explicit Check now, the waiting-to-
be-delivered list on the sender side, and honest reporting when a send could
not be delivered: a closed app is a delay, not a success nobody received.

The setting is off by default and its footer states that checking reveals
app-open times to remembered devices, since that is the reason it is a
setting at all.

Records in the design doc that this shipped as one global toggle rather than
the per-contact opt-in originally specified.

apple/VniDrop/Features/Settings/SettingsModel.swift

@@ -176,6 +176,12 @@ final class SettingsModel: ObservableObject {
 		loadDeviceInfo()
 	}
 
+	/// Surfaces "nothing waiting" from the contacts screen, which has no
+	/// message controller of its own.
+	func reportNothingWaiting() {
+		messages.tryShow(UiMessage(text: .resource(L10n.Contacts.checkNone), tone: .info))
+	}
+
 	func selectSection(_ section: SettingsSection) {
 		state.selectedSection = section
 		if section == .about || section == .bugReport {

apple/VniDrop/Features/Settings/SettingsScreen.swift

@@ -93,7 +93,9 @@ struct SettingsScreen: View {
 		// Contacts brings its own Form and push destination, so it is not wrapped
 		// in the shared section chrome.
 		if section == .contacts {
-			ContactsScreen(model: contacts)
+			ContactsScreen(model: contacts) {
+				model.reportNothingWaiting()
+			}
 		} else {
 			settingsSectionForm(section)
 		}