Windows installer: offer a desktop shortcut (#32) (#184)

The MSI gets a setup wizard (WixUI dialogs, no licence or folder page) with a
Shortcuts page: the Start Menu shortcut as before, plus an opt-in "Create a
desktop shortcut" checkbox bound to DESKTOPSHORTCUT. Silent installs take
DESKTOPSHORTCUT=1. The shortcut goes on the all-users desktop (per-machine
install) in its own component with an HKCU key path (ICE38/43/57), so
uninstall removes it; upgrades read that value back and keep the previous
choice unless DESKTOPSHORTCUT is given.

The build now needs WixToolset.UI.wixext 5.0.2 (release workflow and
package.ps1 updated).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -134,7 +134,9 @@ jobs:
         run: cargo xtask version set "$CADCRAFT_VERSION"
       - name: Install WiX
         shell: pwsh
-        run: dotnet tool install --global wix --version 5.0.2
+        run: |
+          dotnet tool install --global wix --version 5.0.2
+          wix extension add -g WixToolset.UI.wixext/5.0.2
       - name: Build, sign, package
         shell: pwsh
         env: