fix: replace deprecated ListView with CollectionView in TodoApp.MAUI

[adrianhall]

Summary

Replaces the deprecated Microsoft.Maui.Controls.ListView with CollectionView in the TodoApp.MAUI sample's MainPage, eliminating the CS0618 obsolete-API warning surfaced by the new Windows CI (#514):

MainPage.xaml.cs(34,23): warning CS0618: 'ListView' is obsolete: 'ListView is deprecated. Please use CollectionView instead.'

Changes

• MainPage.xaml: replaced <ListView>/<ListView.ItemTemplate> with <CollectionView>/<CollectionView.ItemTemplate>, removing the <ViewCell> wrapper (not used by CollectionView item templates — the DataTemplate content is now the direct visual). Added SelectionMode="Single" and swapped ItemTapped="OnListItemTapped" for SelectionChanged="OnListItemTapped".
• MainPage.xaml.cs: updated OnListItemTapped to accept SelectionChangedEventArgs, reading the tapped item via e.CurrentSelection.FirstOrDefault() instead of e.Item, and cast sender to CollectionView (instead of ListView) to reset SelectedItem = null after handling the tap — preserving the existing "tap to act, then deselect" behavior.
• Resources/Styles/Styles.xaml: removed the now-unused implicit <Style TargetType="ListView"> (SeparatorColor/RefreshControlColor), since ListView is no longer used anywhere in this project and CollectionView doesn't have equivalent properties.

Testing

• dotnet build/dotnet test against the main solution (unaffected, since these are sample-only changes not included in Datasync.Toolkit.sln).
• Verified via the fork's Build Samples CI workflow (workflow_dispatch on issues/515): all sample jobs passed, including todoapp-maui (Android/iOS) and todoapp-windows (which builds the MAUI Windows TFM head — the job that originally surfaced this warning). Confirmed 0 Warning(s) in the MAUI Windows build step logs, versus the previous CS0618 warning.
• Run: https://github.com/adrianhall/CommunityToolkit-Datasync/actions/runs/29148210258

Closes #515