feat(app): check GitHub releases on startup and prompt for updates

Compares the running assembly version against the latest GitHub release
tag on every app launch (desktop and Android) and shows an in-app
overlay with the new version tag when one is available. Best-effort:
network/parse failures are silent, matching the app's existing
non-blocking startup checks.

src/App/Views/MainView.axaml

@@ -1638,5 +1638,34 @@
                 </StackPanel>
             </Border>
         </Border>
+
+        <!-- ============ UPDATE AVAILABLE OVERLAY ============ -->
+        <!-- Same pattern as the help overlay: instant open/close. -->
+        <Border Grid.Row="0" Grid.RowSpan="3"
+                Background="{DynamicResource ScrimBrush}"
+                Tapped="OnUpdateAvailableOverlayBackdropTapped"
+                IsVisible="{Binding IsUpdateAvailableOpen}">
+            <Border Background="{DynamicResource OverlayCardBrush}"
+                    BorderBrush="{DynamicResource BorderSubtleBrush}" BorderThickness="1" CornerRadius="8"
+                    MaxWidth="440" Margin="16"
+                    HorizontalAlignment="Center" VerticalAlignment="Center">
+                <StackPanel Margin="24" Spacing="16">
+                    <TextBlock Text="{Binding Loc[update.title]}"
+                               FontSize="18" FontWeight="Bold"/>
+                    <TextBlock TextWrapping="Wrap">
+                        <Run Text="{Binding Loc[update.message]}"/>
+                        <Run Text=" "/>
+                        <Run Text="{Binding UpdateAvailableTag}" FontWeight="Bold"/>
+                    </TextBlock>
+                    <StackPanel Orientation="Horizontal" Spacing="8" HorizontalAlignment="Right">
+                        <Button Content="{Binding Loc[update.download]}"
+                                Classes="accent"
+                                Click="OnOpenReleasePageClick"/>
+                        <Button Content="{Binding Loc[update.dismiss]}"
+                                Command="{Binding CloseUpdateAvailableCommand}"/>
+                    </StackPanel>
+                </StackPanel>
+            </Border>
+        </Border>
     </Grid>
 </UserControl>