Group tasks feature (#3)

This PR contains the work done to implement the **Group Task** feature, and to integrate it into the main content view of the app. Furthermore, an issue related to triggering a view update after inserting the todo tasks into the model container has been fixed.

Reviewed-on: rock-n-code/todos-app#3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

App/Sources/View Models/ContentViewModel.swift

@@ -11,7 +11,7 @@ extension ContentView {
     /// The view model manages:
     /// - Presentation state for the new task view within a sheet.
     @Observable
-    class ViewModel {
+    final class ViewModel {
 
         // MARK: Properties
 

App/Sources/View Models/NewTaskViewModel.swift

@@ -13,7 +13,7 @@ extension NewTaskView {
     /// - Form validation (ensuring title is not empty)
     /// - Saving the todo to SwiftData model context
     @Observable
-    class ViewModel {
+    final class ViewModel {
 
         // MARK: Properties
 

App/Sources/View Models/TaskGroupListViewModel.swift

@@ -0,0 +1,87 @@
+import SwiftData
+import SwiftUI
+
+extension TaskGroupListView {
+
+    /// View model for managing the logic for the task group list view.
+    ///
+    /// This observable class encapsulates the state and business logic for the task group list view.
+    /// It follows the MVVM pattern to separate presentation logic from the view layer.
+    ///
+    /// ## Responsibilities
+    /// - Providing access to all task groups for iteration
+    /// - Efficiently counting tasks per group using SwiftData fetch descriptors
+    /// - Managing date calculations for group categorization
+    @Observable
+    final class ViewModel {
+
+        // MARK: Properties
+
+        /// The calendar used for date calculations.
+        ///
+        /// This calendar is used to determine day boundaries for categorizing tasks into date groups: today, upcoming, overdue.
+        /// Defaults to the current calendar.
+        private let calendar: Calendar
+
+        // MARK: Initialization
+
+        /// Initializes a new view model instance.
+        ///
+        /// - Parameter calendar: The calendar to use for date calculations. Defaults to `.current`.
+        init(calendar: Calendar = .current) {
+            self.calendar = calendar
+        }
+
+        // MARK: Computed
+
+        /// Returns all available task groups in an predetermined order.
+        ///
+        /// This computed property provides access to all task group cases for iteration in the view.
+        ///
+        /// - Returns: An array of all `TodoGroup` cases in a predetermined order.
+        var groups: [TodoGroup] {
+            TodoGroup.allCases
+        }
+
+        // MARK: Functions
+
+        /// Calculates the number of tasks for a given task group from an in-memory collection.
+        ///
+        /// This method filters the provided tasks array to count only those tasks that belong
+        /// to the specified group based on their due date and completion status. Tasks are
+        /// categorized as follows:
+        ///
+        /// - **Completed**: Tasks with a non-nil `completed` date
+        /// - **Overdue**: Incomplete tasks due before today
+        /// - **Today**: Incomplete tasks due today (from start of day to end of day)
+        /// - **Upcoming**: Incomplete tasks due tomorrow or later
+        ///
+        /// - Parameters:
+        ///   - tasks: The array of tasks to filter and count.
+        ///   - group: The task group to count tasks for.
+        /// - Returns: The number of tasks in the specified group.
+        func count(
+            _ tasks: [Todo],
+            for group: TodoGroup
+        ) -> Int {
+            let today = calendar.startOfDay(for: .now)
+            let tomorrow = calendar.date(byAdding: .day, value: 1, to: today) ?? .now
+
+            return tasks
+                .filter { task in
+                    switch group {
+                        case .completed:
+                            task.completed != nil
+                        case .overdue:
+                            task.completed == nil && task.due < today
+                        case .today:
+                            task.completed == nil && task.due >= today && task.due < tomorrow
+                        case .upcoming:
+                            task.completed == nil && task.due >= tomorrow
+                    }
+                }
+                .count
+        }
+
+    }
+}