Add OmniHost desktop shell, VSCode-style chrome, and multi-OS packaging workflow

[Copilot]

Wrap the existing LVGL Editor web app as an OmniHost desktop application and add CI packaging for Linux, macOS, and Windows artifacts. The desktop shell follows the OmniHost cross-platform pattern and adopts VSCode-style window chrome/menu behavior.

• Desktop host

  • Add desktop/LvglEditor.Desktop as the OmniHost entrypoint for the editor.
  • Use cross-platform runtime selection:
    • Windows: Win32Runtime + WebView2
    • Linux: GtkRuntime + WebKitGtk
    • macOS: explicit unsupported-path placeholder to keep artifact flow aligned until OmniHost exposes a native runtime
  • Serve the built Vite output from wwwroot via app://localhost/....
  • Configure VSCode-style host chrome with built-in title bar controls (minimize / maximize / close).

• Editor chrome / menu integration

  • Add a VSCode-style in-app menu bar for 文件 / 编辑 / 视图 / 帮助.
  • Wire menu actions to existing editor commands: new/open/save/export/import, undo/redo, tab switching, resource panel toggle, settings, help.
  • Keep the existing editor toolbar and tab model intact while making the shell desktop-oriented.

• Build and packaging

  • Add build:desktop-web to generate the static frontend bundle consumed by the desktop host.
  • Add desktop-packages.yml to publish zipped desktop artifacts for Linux, macOS, and Windows.
  • Add NuGet.config for OmniHost package restore from the maikebing GitHub Packages feed.
  • Gate packaging steps on feed credentials so CI can clearly no-op when the package token is not configured.

• Docs / ops

  • Document desktop build flow and required package-feed credentials.
  • Ignore generated .NET build artifacts.

var builder = OmniApp.CreateBuilder(args)
    .Configure(options =>
    {
        options.Title = "LVGL Editor";
        options.CustomScheme = "app";
        options.ContentRootPath = Path.Combine(AppContext.BaseDirectory, "wwwroot");
        options.StartUrl = "app://localhost/index.html";
        options.StartMaximized = true;
        options.BuiltInTitleBarStyle = OmniBuiltInTitleBarStyle.VsCode;
        options.WindowStyle = OperatingSystem.IsWindows()
            ? OmniWindowStyle.VsCode
            : OmniWindowStyle.Frameless;
    });