refactor(app): replace nested settings submenu with in-app overlay

The Impostazioni menu now opens a single overlay (same pattern as
address/server overlays) instead of nested OS popup menus, which are
slow to respond under WSLg.

The overlay groups language (RadioButton) and unit (RadioButton) in one
panel, with a button to open the server overlay. Esc and backdrop tap
close it. OpenServerSettings() closes the settings overlay first so the
two overlays never stack.

src/App/ViewModels/MainWindowViewModel.cs

@@ -231,16 +231,31 @@ public partial class MainWindowViewModel : ViewModelBase
     [ObservableProperty]
     private bool useSsl = true;
 
-    /// <summary>Overlay impostazioni server: aperto da menu Impostazioni → Server.</summary>
+    /// <summary>Overlay impostazioni server: aperto dall'overlay Impostazioni.</summary>
     [ObservableProperty]
     private bool isServerSettingsOpen;
 
     [RelayCommand]
-    private void OpenServerSettings() => IsServerSettingsOpen = true;
+    private void OpenServerSettings()
+    {
+        IsSettingsOpen = false;
+        IsServerSettingsOpen = true;
+    }
 
     [RelayCommand]
     private void CloseServerSettings() => IsServerSettingsOpen = false;
 
+    /// <summary>Overlay impostazioni (lingua, unità, server): in-app per evitare
+    /// i popup di menu annidati, lenti su WSLg.</summary>
+    [ObservableProperty]
+    private bool isSettingsOpen;
+
+    [RelayCommand]
+    private void OpenSettings() => IsSettingsOpen = true;
+
+    [RelayCommand]
+    private void CloseSettings() => IsSettingsOpen = false;
+
     [ObservableProperty]
     private string connectionStatus = Loc.Tr("conn.none");