Address AppImage packaging warnings

[byrongamatos]

Restored from slopsmith/slopsmith#124 — original issue, opened by @mogul on 2026-05-01.
[restored-from: slopsmith/slopsmith#124]

When the AppImage is being created in the Linux pipeline, errors indicate that the .desktop file isn't set up correctly and metadata is missing.

$ /tmp/appimagetool /tmp/appimage_build/squashfs-root ~/bin/Slopsmith-0.2.2-fixed2.AppImage 2>&1
  appimagetool, continuous build (git version 8c8c91f), build 295 built on 2025-12-04 17:56:36 UTC
  /tmp/appimage_build/squashfs-root/slopsmith-desktop.desktop: error: (will be fatal in the future): value item "Audio" in key "Categories" in group "Desktop Entry" requires another category to be present among the following categories: AudioVideo
  Using architecture x86_64
  /tmp/appimage_build/squashfs-root should be packaged as /home/mogul/bin/Slopsmith-0.2.2-fixed2.AppImage
  WARNING: AppStream upstream metadata is missing, please consider creating it
       in usr/share/metainfo/slopsmith-desktop.appdata.xml
       Please see https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps
       for more information or use the generator at
       https://docs.appimage.org/packaging-guide/optional/appstream.html#using-the-appstream-generator
  Generating squashfs...