docs: explain systemd desktop detection

[TyceHerrman]

Summary

Document how Satpaper uses XDG_CURRENT_DESKTOP when selecting its built-in Linux wallpaper handler, especially when it runs as a systemd user service.

Changes

• list the exact XDG_CURRENT_DESKTOP values recognized for GNOME, Ubuntu GNOME, and KDE
• show how to inspect the value from a graphical-session terminal
• explain how to pass an inherited supported value to a systemd user unit and restart it
• clarify that unsupported desktops such as Cinnamon should use --wallpaper-command/SATPAPER_WALLPAPER_COMMAND instead of claiming GNOME or KDE
• note that a custom wallpaper command bypasses desktop detection

Testing

• documentation-only review against the current desktop-detection behavior
• git diff --check

Closes #27