Add Linux metadata files

[NN708]

This PR introduces standard Linux metadata files, including XDG desktop entry and AppStream MetaInfo file.

These files are essential for Linux packaging, particularly for Flatpak. This PR follows the Flathub quality guidelines, such as keeping the summary under 35 characters. Since I didn't find an official screenshot, I've used one from the release notes. If an official screenshot is available, feel free to replace it.

Related to #1511. Supersedes #549.