feat(ui): two-column desktop layout for Send/Receive, fixed window size

Send tab: Recipient and Amount cards side-by-side on desktop using a
two-column Grid, making full use of the available window width.
Summary card and action buttons remain full-width below both columns.
Mobile retains the original vertical stack.

Receive tab: QR card on the left (auto-width), address card expands
to fill the right column on desktop — standard fintech wallet pattern.
Mobile retains QR above, address below.

Window enlarged to 1020×680 (from 900×620) and fixed with CanResize=False;
Min/Max dimensions match so the resize handle never appears.

src/App/Views/MainWindow.axaml

@@ -4,11 +4,14 @@
         xmlns:views="using:PalladiumWallet.App.Views"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
-        mc:Ignorable="d" d:DesignWidth="900" d:DesignHeight="620"
+        mc:Ignorable="d" d:DesignWidth="1020" d:DesignHeight="680"
         x:Class="PalladiumWallet.App.Views.MainWindow"
         x:DataType="vm:MainWindowViewModel"
         Icon="/Assets/logo.ico"
-        Width="900" Height="620"
+        Width="1020" Height="680"
+        MinWidth="1020" MinHeight="680"
+        MaxWidth="1020" MaxHeight="680"
+        CanResize="False"
         Title="{Binding WindowTitle}">
 
     <Design.DataContext>