Add AppStream stock icon and install metainfo/icon

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Vitexus

debian/io.github.spoje_net.cnb_cache.metainfo.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop-application">
   <id>io.github.spoje_net.cnb_cache</id>
+  <icon type="stock">cnb-cache</icon>
   <metadata_license>MIT</metadata_license>
   <project_license>MIT</project_license>
   <name>cnb-cache</name>