Issue #18: Windows MSI: finish dialog with Launch LightCraft

Interactive MSI installs now walk Welcome -> Ready to install -> progress
-> "Completed the LightCraft Setup Wizard" (Finish) with "LightCraft has
been installed" text and a checked-by-default "Launch LightCraft"
checkbox. Failure and cancellation end on WiX's standard FatalError /
UserExit pages; silent (/qn) and basic-UI (/passive) installs show no
dialogs and launch nothing.

- lightcraft.wxs: custom UI set = WixUI_Minimal without the licence page
  (MIT OR Apache-2.0 asks nothing of installers), maintenance dialogs for
  repair/remove, ARPNOMODIFY. Launch uses the Util extension's
  WixUnelevatedShellExec via Explorer (runs as the signed-in user, never
  elevated), Return="ignore", only when NOT Installed and the box is ticked.
- package.ps1: add WixToolset.UI/Util.wixext 5.0.2 to the global cache,
  pass -ext ... -culture en-us, and generate plain side/banner bitmaps
  (brand blue / white) so the MSI carries no WiX sample artwork.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -127,6 +127,7 @@ jobs:
         if: inputs.version != ''
         shell: bash
         run: cargo xtask version set "$LIGHTCRAFT_VERSION"
+      # package.ps1 adds the matching WixToolset.UI/Util.wixext (5.0.2) to the global extension cache.
       - name: Install WiX
         shell: pwsh
         run: dotnet tool install --global wix --version 5.0.2