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/Previews/Modifiers/EmptyContainerModifier.swift

@@ -0,0 +1,79 @@
+import SwiftData
+import SwiftUI
+
+/// A preview modifier that provides an empty SwiftData model container for SwiftUI previews.
+///
+/// This modifier creates an in-memory SwiftData model container with no data, useful for testing and previewing views in their empty or initial state.
+/// It's particularly helpful for testing empty state UI, loading states, or views that should gracefully handle no data.
+///
+/// ## Usage
+/// Use the `.emptyContainer` trait on your preview:
+///
+/// ```swift
+/// #Preview(traits: .emptyContainer) {
+///     TaskGroupListView()
+/// }
+/// ```
+///
+/// This automatically provides an empty model container to the preview, allowing SwiftData queries to return empty results.
+struct EmptyContainerModifier: PreviewModifier {
+    
+    // MARK: Functions
+    
+    /// Creates and configures a shared in-memory model container with no data.
+    ///
+    /// This method is called once when the preview is loaded and creates an in-memory
+    /// SwiftData container with no persisted entities. The container is configured
+    /// to store data only in memory, ensuring previews don't affect any persistent storage.
+    ///
+    /// - Returns: An empty `ModelContainer` configured for in-memory storage.
+    /// - Throws: An error if the model container cannot be created.
+    static func makeSharedContext() async throws -> ModelContainer {
+        try ModelContainer(
+            for: Todo.self,
+            configurations: .init(isStoredInMemoryOnly: true)
+        )
+    }
+    
+    /// Applies the empty model container to the preview content.
+    ///
+    /// This method is called by SwiftUI to apply the modifier to the preview.
+    /// It injects the empty model container into the SwiftUI environment, making it
+    /// available to any views that use `@Query` or `@Environment(\.modelContext)`.
+    /// Queries will return empty results, allowing you to test empty state behavior.
+    ///
+    /// - Parameters:
+    ///   - content: The preview content to modify.
+    ///   - context: The empty model container.
+    /// - Returns: The content with the empty model container applied.
+    func body(
+        content: Content,
+        context: ModelContainer
+    ) -> some View {
+        content.modelContainer(context)
+    }
+    
+}
+
+// MARK: - PreviewTrait
+
+extension PreviewTrait where T == Preview.ViewTraits {
+    
+    /// A preview trait that provides an empty model container.
+    ///
+    /// Use this trait to test views in their empty state without any data.
+    /// This is particularly useful for testing empty state UI, placeholder content,
+    /// and ensuring views handle the absence of data gracefully.
+    ///
+    /// ## Example
+    /// ```swift
+    /// #Preview("Empty State", traits: .emptyContainer) {
+    ///     TaskGroupListView()
+    /// }
+    /// ```
+    ///
+    /// The view will receive an empty SwiftData container, causing any `@Query`
+    /// properties to return empty collections.
+    static var emptyContainer: Self = .modifier(EmptyContainerModifier())
+    
+}