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/io.github.HakanSeven12.OpenCadStudio.metainfo.xml

@@ -0,0 +1,50 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- AppStream metadata: integrates the AppImage with Linux software centres
+     (name, summary, icon, categories, screenshots). Installed into
+     /usr/share/metainfo/ inside the AppDir by the release workflow. -->
+<component type="desktop-application">
+  <id>io.github.HakanSeven12.OpenCadStudio</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-3.0-or-later</project_license>
+
+  <name>Open CAD Studio</name>
+  <summary>2D/3D CAD application for DWG and DXF drawings</summary>
+
+  <description>
+    <p>
+      Open CAD Studio is a 2D/3D CAD application for creating and editing
+      technical drawings. It reads and writes the standard DWG and DXF file
+      formats so drawings stay compatible with other CAD software.
+    </p>
+  </description>
+
+  <launchable type="desktop-id">io.github.HakanSeven12.OpenCadStudio.desktop</launchable>
+
+  <categories>
+    <category>Graphics</category>
+    <category>Engineering</category>
+  </categories>
+
+  <keywords>
+    <keyword>CAD</keyword>
+    <keyword>DWG</keyword>
+    <keyword>DXF</keyword>
+    <keyword>drawing</keyword>
+    <keyword>design</keyword>
+    <keyword>engineering</keyword>
+  </keywords>
+
+  <url type="homepage">https://github.com/HakanSeven12/OpenCADStudio</url>
+  <url type="bugtracker">https://github.com/HakanSeven12/OpenCADStudio/issues</url>
+
+  <developer id="io.github.hakanseven12">
+    <name>Hakan Seven</name>
+  </developer>
+
+  <content_rating type="oars-1.1" />
+
+  <provides>
+    <mediatype>image/vnd.dwg</mediatype>
+    <mediatype>image/vnd.dxf</mediatype>
+  </provides>
+</component>