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/BoolToTabPlacementConverter.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Globalization;
+using Avalonia.Controls;
+using Avalonia.Data.Converters;
+
+namespace PalladiumWallet.App.ViewModels;
+
+/// <summary>
+/// true (mobile) → Dock.Bottom — tab strip in basso, standard Android.
+/// false (desktop) → Dock.Top — comportamento predefinito Avalonia.
+/// </summary>
+public sealed class BoolToTabPlacementConverter : IValueConverter
+{
+    public static readonly BoolToTabPlacementConverter Instance = new();
+
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture) =>
+        value is true ? Dock.Bottom : Dock.Top;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture) =>
+        throw new NotSupportedException();
+}