Fix tab selection timing to wait for account sign-in

Tab selection was being set immediately during app configuration, before
the user account had completed sign-in. This caused tabs that require
authentication (like Subscriptions and Playlists) to not be properly
selected on startup since they weren't visible yet.

Changes:
- Add notification system for account configuration completion
- Post notification after all account types finish configuration:
  * Accounts with existing tokens
  * Accounts requiring sign-in (after network request completes)
  * Anonymous/public accounts
  * Error cases (missing credentials, network failures)
- Set up observer before account configuration to ensure notification
  is received
- Set tab selection only when account is fully configured

Extensions/Notification+Names.swift

@@ -0,0 +1,5 @@
+import Foundation
+
+extension Notification.Name {
+    static let accountConfigurationComplete = Notification.Name("accountConfigurationComplete")
+}