feat(ui): responsive layout for portrait mobile (Android)

Add IsMobile property and BoolToTabPlacementConverter; wire
TabStripPlacement to move tabs to the bottom on mobile (standard
Android pattern). Hide the desktop menu bar on mobile and expose
Settings/Help via a compact header row instead.

Replace fixed-width multi-column grids with dual desktop/mobile
templates in History, Addresses and Contacts lists; adapt Send,
Receive and the add-contact form for narrow screens. Remove hard-coded
Width from all five overlays (560–640 px) in favour of MaxWidth +
Margin="16" so they never overflow a 360 px screen; add ScrollViewer
to Address info, Server settings and Settings overlays.

Desktop layout is unchanged: all differences are gated on the
IsMobile/IsDesktop bool, which is a platform constant (false on desktop).

src/App/ViewModels/MainWindowViewModel.cs

@@ -94,6 +94,8 @@ public partial class MainWindowViewModel : ViewModelBase
     /// <summary>true su desktop; false su Android/iOS — nasconde le funzioni legate al filesystem libero.</summary>
     public bool IsDesktop => !OperatingSystem.IsAndroid() && !OperatingSystem.IsIOS();
 
+    public bool IsMobile => !IsDesktop;
+
     public string UnitLabel => _config.Unit;
     public AppConfig CurrentConfig => _config;