Xodus Nexus backend 0.2.12

• Add Xbox friends and messaging support, including authenticated message sending and server-confirmed send results.
• Support downloadable add-ons associated with an installed base game.
• Preserve conditional license information used to display games correctly in the library.
• Explain Microsoft Store device-limit refusals instead of reporting an unexplained licensing failure.
• Support declared plaintext game executables and launching from filesystems without symbolic links.
• Provide package download and installation-size information.