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

Shared/YatteeApp.swift

@@ -162,6 +162,23 @@ struct YatteeApp: App {
 
             SDWebImageManager.defaultImageCache = PINCache(name: "stream.yattee.app")
 
+            NotificationCenter.default.addObserver(
+                forName: .accountConfigurationComplete,
+                object: nil,
+                queue: .main
+            ) { _ in
+                let startupSection = Defaults[.startupSection]
+                var section: TabSelection? = startupSection.tabSelection
+
+                #if os(macOS)
+                    if section == .playlists {
+                        section = .search
+                    }
+                #endif
+
+                NavigationModel.shared.tabSelection = section ?? .search
+            }
+
             if !Defaults[.lastAccountIsPublic] {
                 AccountsModel.shared.configureAccount()
             }
@@ -180,17 +197,6 @@ struct YatteeApp: App {
                 }
             }
 
-            let startupSection = Defaults[.startupSection]
-            var section: TabSelection? = startupSection.tabSelection
-
-            #if os(macOS)
-                if section == .playlists {
-                    section = .search
-                }
-            #endif
-
-            NavigationModel.shared.tabSelection = section ?? .search
-
             DispatchQueue.main.async {
                 playlists.load()
             }