Replace onboarding flow with silent v1 import and iCloud alert

Delete the multi-page onboarding sheet. On first launch the app now
silently imports any v1 instances from UserDefaults (splitting embedded
basic-auth credentials out of the URL and into the Keychain) and then,
if the device is signed in to iCloud, shows a single alert offering to
enable sync. Accepting shows a blocking progress overlay until the
initial upload completes.

Yattee/Core/AppEnvironment.swift

@@ -277,6 +277,7 @@ final class AppEnvironment {
         // Initialize Legacy Migration Service
         self.legacyMigrationService = LegacyDataMigrationService(
             instancesManager: instances,
+            basicAuthCredentialsManager: basicAuthCreds,
             httpClient: client
         )
 

Yattee/Core/Notifications.swift

@@ -0,0 +1,13 @@
+//
+//  Notifications.swift
+//  Yattee
+//
+//  App-wide notification names.
+//
+
+import Foundation
+
+extension Notification.Name {
+    static let showSettings = Notification.Name("showSettings")
+    static let showOpenLinkSheet = Notification.Name("showOpenLinkSheet")
+}