feat(ui): Help overlay with Info/Donate tabs, fixed dimensions

Help button now shows a fixed-size two-tab overlay:

Info tab: app name + version, one-line description, thin separator,
  © 2026 Davide Grilli — MIT License, Built with .NET 10 · Avalonia 12 · NBitcoin.

Donate tab: dev address (plm1qdq3…) read-only display, free amount
  input, prepare-preview row, confirm-and-broadcast button.

Overlay dimensions are stable across tabs and between desktop/mobile:
- TabControl Height=320 (fixed, never resizes on tab switch)
- HorizontalAlignment=Stretch + MaxWidth=440 on the card (always fills
  available width; previously Center caused the card to shrink to content
  width, which differed between Info and Donate)
- Each tab content wrapped in ScrollViewer for safe overflow handling

New MainWindowViewModel.Donate.cs: PrepareDonate / ConfirmDonate /
ResetDonate (called on overlay close). Six-language Loc keys added for
all donate strings.

src/App/Views/MainView.axaml

@@ -1373,14 +1373,16 @@
             <Border Background="{DynamicResource OverlayCardBrush}"
                     BorderBrush="{DynamicResource BorderSubtleBrush}" BorderThickness="1" CornerRadius="8"
                     MaxWidth="440" Margin="16"
-                    HorizontalAlignment="Center" VerticalAlignment="Center">
+                    HorizontalAlignment="Stretch" VerticalAlignment="Center">
                 <StackPanel Margin="24" Spacing="16">
                     <TextBlock Text="{Binding Loc[help.title]}"
                                FontSize="18" FontWeight="Bold"/>
 
-                    <TabControl>
+                    <!-- Fixed height so the overlay never resizes when switching tabs. -->
+                    <TabControl Height="320">
                         <!-- Tab: Info -->
                         <TabItem Header="{Binding Loc[help.tab.info]}">
+                            <ScrollViewer VerticalScrollBarVisibility="Auto">
                                 <StackPanel Spacing="12" Margin="0,12,0,0">
                                     <StackPanel Spacing="4">
                                         <TextBlock Text="Palladium Wallet" FontSize="16" FontWeight="Bold"/>
@@ -1388,11 +1390,23 @@
                                                    Foreground="{DynamicResource TextSecondaryBrush}"/>
                                     </StackPanel>
                                     <TextBlock Text="{Binding Loc[help.info]}" TextWrapping="Wrap"/>
+                                    <Border BorderBrush="{DynamicResource BorderSubtleBrush}"
+                                            BorderThickness="0,1,0,0" Margin="0,4,0,0"/>
+                                    <StackPanel Spacing="4">
+                                        <TextBlock Text="© 2026 Davide Grilli — MIT License"
+                                                   FontSize="12"
+                                                   Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                        <TextBlock Text="Built with .NET 10 · Avalonia 12 · NBitcoin"
+                                                   FontSize="11"
+                                                   Foreground="{DynamicResource TextSecondaryBrush}"/>
                                     </StackPanel>
+                                </StackPanel>
+                            </ScrollViewer>
                         </TabItem>
 
                         <!-- Tab: Donate -->
                         <TabItem Header="{Binding Loc[help.tab.donate]}">
+                            <ScrollViewer VerticalScrollBarVisibility="Auto">
                                 <StackPanel Spacing="12" Margin="0,12,0,0">
                                     <TextBlock Text="{Binding Loc[donate.desc]}" TextWrapping="Wrap"
                                                Foreground="{DynamicResource TextSecondaryBrush}"/>
@@ -1438,6 +1452,7 @@
                                                 Command="{Binding PrepareDonateCommand}"/>
                                     </StackPanel>
                                 </StackPanel>
+                            </ScrollViewer>
                         </TabItem>
                     </TabControl>