Add --install-desktop to install a desktop entry

Detects an installed terminal emulator (kitty, alacritty, wezterm, foot,
ghostty, gnome-terminal, konsole, xfce4-terminal, tilix,
x-terminal-emulator, xterm) and bakes an explicit launch command into
the entry, since Terminal=true is unreliable on modern desktops.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

assets/emby-client.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Emby Client
+GenericName=Emby TUI Client
+Comment=TUI Emby client with libmpv playback
+Exec={EXEC}
+Terminal={TERMINAL}
+Categories=AudioVideo;Player;
+Keywords=emby;media;player;tui;