feat(packaging): desktop shortcut, icon-cache refresh, checksums, AppStream

Windows installer:
- Optional Desktop shortcut feature (default on, deselectable) via a switch to
  the WixUI_FeatureTree UI.
- Refresh the shell after install (ie4uinit -show) so .dwg/.dxf icons and
  associations appear without a reboot.
- Tidy the Add/Remove Programs entry: ARPNOREPAIR plus contact/update/comments
  metadata (Modify stays enabled for the feature tree).

Release workflow:
- Publish SHA256 sidecars for the AppImage, portable .exe, MSI and .dmg.

Linux:
- Ship an AppStream metainfo.xml for software-centre integration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/windows/ui.wxs

@@ -6,7 +6,9 @@
   (which needs `-ext WixUIExtension` for the WixUI_Minimal dialog set).
 
   Provides:
-    * WixUI_Minimal wizard (Welcome → Progress → Finish).
+    * WixUI_FeatureTree wizard (Welcome → License → Features → Progress →
+      Finish). The Features page lets the user deselect the optional Desktop
+      shortcut feature (default on).
     * A pre-ticked "Launch Open CAD Studio" checkbox on the finish screen
       that starts the freshly installed exe when the user clicks Finish.
 -->
@@ -41,7 +43,7 @@
       Impersonate='yes' />
 
     <UI Id='OpenCADStudioUI'>
-      <UIRef Id='WixUI_Minimal' />
+      <UIRef Id='WixUI_FeatureTree' />
       <Publish
         Dialog='ExitDialog'
         Control='Finish'