Updated the ContentView view in the app target to support navigation split view and to show floating new task button on both sidebar and details views when required.

App/Sources/View Models/ContentViewModel.swift

@@ -10,6 +10,7 @@ extension ContentView {
     ///
     /// The view model manages:
     /// - Presentation state for the new task view within a sheet.
+    /// - Selected task group for the navigation split view.
     @Observable
     final class ViewModel {
 
@@ -18,22 +19,60 @@ extension ContentView {
         /// Whether the new task view should be shown.
         var newTaskShown: Bool
 
+        /// The currently selected todo task group in the navigation split view.
+        ///
+        /// When a user selects a group from the sidebar, this property is updated
+        /// to reflect the selection. The detail view displays content based on this value.
+        var group: TodoGroup?
+
         // MARK: Initialization
 
         /// Initializes a new view model instance with default values.
         ///
         /// The view model is initialized with:
         /// - New task view hidden
+        /// - No task group selected
         init() {
             newTaskShown = false
+            group = nil
+        }
+
+        // MARK: Computed
+
+        /// Determines whether the new task button should be shown in the detail view.
+        ///
+        /// This prevents showing the add button when viewing completed tasks,
+        /// as users should not create new tasks in the completed state.
+        var showNewTaskButtonInDetails: Bool {
+            group != nil
+            && group != .completed
         }
 
         // MARK: Functions
 
         /// Shows the new task view by setting the presentation state to true.
+        ///
+        /// This method triggers the presentation of a sheet where users can create a new task.
+        /// Called when the floating action button is tapped.
         func showNewTask() {
             newTaskShown = true
         }
 
+        /// Determines whether the new task button should be shown in the sidebar.
+        ///
+        /// The button appears in the sidebar only on compact horizontal size class devices
+        /// (typically iPhones in portrait mode). This provides a way to create tasks when
+        /// the sidebar is displayed as a full screen view.
+        ///
+        /// - Parameter horizontalSizeClass: The current horizontal size class of the environment.
+        /// - Returns: `true` if the button should be shown in the sidebar, `false` otherwise.
+        func showNewTaskButtonInSidebar(when horizontalSizeClass: UserInterfaceSizeClass?) -> Bool {
+            guard let horizontalSizeClass else {
+                return false
+            }
+
+            return horizontalSizeClass == .compact
+        }
+
     }
 }