feat(app): version in window title and Help overlay

- csproj: add <Version>0.9.0</Version> as single source of truth
- AppVersion reads Major.Minor.Build from the assembly; WindowTitle
  exposes "Palladium Wallet 0.9.0" bound to Window.Title
- Menu Help → in-app overlay (same pattern as settings/server overlays)
  showing app name, version and a short description; backdrop tap and
  Esc close it
- i18n: menu.help, help.title, help.info in all 6 languages

src/App/Views/MainWindow.axaml

@@ -9,7 +9,7 @@
         x:DataType="vm:MainWindowViewModel"
         Icon="/Assets/logo.ico"
         Width="900" Height="620"
-        Title="Palladium Wallet">
+        Title="{Binding WindowTitle}">
 
     <Design.DataContext>
         <vm:MainWindowViewModel/>
@@ -28,6 +28,8 @@
             </MenuItem>
             <MenuItem Header="{Binding Loc[menu.settings]}"
                       Command="{Binding OpenSettingsCommand}"/>
+            <MenuItem Header="{Binding Loc[menu.help]}"
+                      Command="{Binding OpenHelpCommand}"/>
         </Menu>
 
         <!-- ============ WIZARD DI SETUP (§15): un passo alla volta ============ -->
@@ -714,5 +716,33 @@
                 </StackPanel>
             </Border>
         </Border>
+
+        <!-- ============ OVERLAY HELP / INFORMAZIONI ============ -->
+        <!-- Stesso pattern dell'overlay impostazioni: apertura/chiusura istantanee. -->
+        <Border Grid.Row="0" Grid.RowSpan="3"
+                Background="#99000000"
+                Tapped="OnHelpOverlayBackdropTapped"
+                IsVisible="{Binding IsHelpOpen}">
+            <Border Background="{DynamicResource SystemControlBackgroundChromeMediumLowBrush}"
+                    BorderBrush="Gray" BorderThickness="1" CornerRadius="8"
+                    Width="460"
+                    HorizontalAlignment="Center" VerticalAlignment="Center">
+                <StackPanel Margin="24" Spacing="16">
+                    <TextBlock Text="{Binding Loc[help.title]}"
+                               FontSize="18" FontWeight="Bold"/>
+
+                    <StackPanel Spacing="4">
+                        <TextBlock Text="Palladium Wallet" FontSize="16" FontWeight="Bold"/>
+                        <TextBlock Text="{Binding WindowTitle}" FontSize="12" Foreground="Gray"/>
+                    </StackPanel>
+
+                    <TextBlock Text="{Binding Loc[help.info]}" TextWrapping="Wrap"/>
+
+                    <Button Content="{Binding Loc[addr.close]}"
+                            HorizontalAlignment="Right"
+                            Command="{Binding CloseHelpCommand}"/>
+                </StackPanel>
+            </Border>
+        </Border>
     </Grid>
 </Window>