feat: add CMD+Q keyboard shortcut to Exit menu on macOS

[sixzjd]

Summary

• Add Gesture = "CMD+Q" to the Exit NativeMenuItem in the macOS application menu
• This enables the standard Cmd+Q keyboard shortcut for quitting the application

Problem

On macOS, pressing Cmd+Q does nothing because:

1. DisableDefaultApplicationMenuItems = true removes the standard macOS Quit menu
2. The replacement Exit NativeMenuItem has no Gesture property set

Fix

One-line change in src/BD.WTTS.Client.Avalonia/UI/App.axaml.cs:

- menus.Add(new NativeMenuItem { Header = Strings.Exit, Command = ReactiveCommand.Create(() => { Shutdown(); }) });
+ menus.Add(new NativeMenuItem { Header = Strings.Exit, Gesture = "CMD+Q", Command = ReactiveCommand.Create(() => { Shutdown(); }) });

Note: The DebugWindow.axaml already uses Gesture="CMD+Q" pattern (line 23), so this is consistent with the existing codebase.