feat(ui): replace mobile tab strip with custom full-width nav bar

Avalonia's TabStrip ignores HorizontalAlignment overrides when placed at
Bottom, causing the items to cluster left. Replace it with a dedicated
Grid (ColumnDefinitions="*,*,*,*,*") that guarantees 5 equal columns
across the full width. The built-in TabStrip is hidden on mobile; buttons
in the custom bar call SelectTabCommand to drive SelectedTabIndex on the
TabControl.

src/App/ViewModels/MainWindowViewModel.cs

@@ -117,6 +117,12 @@ public partial class MainWindowViewModel : ViewModelBase
     [ObservableProperty]
     private string statusMessage = "";
 
+    [ObservableProperty]
+    private int selectedTabIndex;
+
+    [RelayCommand]
+    private void SelectTab(string index) => SelectedTabIndex = int.Parse(index);
+
     // ---- collections per la dashboard ----
 
     public ObservableCollection<HistoryRow> History { get; } = [];