Multiplatform UI support fixes

Model/NavigationState.swift

@@ -0,0 +1,43 @@
+import Foundation
+import SwiftUI
+
+final class NavigationState: ObservableObject {
+    @Published var tabSelection: TabSelection = .subscriptions
+
+    @Published var showingChannel = false
+    @Published var channel: Channel?
+
+    @Published var showingVideoDetails = false
+    @Published var video: Video?
+
+    func openChannel(_ channel: Channel) {
+        self.channel = channel
+        showingChannel = true
+    }
+
+    func closeChannel() {
+        showingChannel = false
+        channel = nil
+    }
+
+    func openVideoDetails(_ video: Video) {
+        self.video = video
+        showingVideoDetails = true
+    }
+
+    func closeVideoDetails() {
+        showingVideoDetails = false
+        video = nil
+    }
+
+    var tabSelectionOptionalBinding: Binding<TabSelection?> {
+        Binding<TabSelection?>(
+            get: {
+                self.tabSelection
+            },
+            set: {
+                self.tabSelection = $0 ?? .subscriptions
+            }
+        )
+    }
+}