Install ChatGPT as a desktop app under Install > AI (#6747)

Drops the ChatGPT web app from the default set at the same time, so installing
the openai-codex-desktop package can't leave two identical-looking ChatGPT
entries in the launcher. Super + Shift + A still opens the web version, which
is the only place it was really used.

The bundled ChatGPT icon stays: the package's own chatgpt.desktop asks for
Icon=chatgpt and ships no hicolor icon of its own.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

applications/ChatGPT.desktop

@@ -1,8 +0,0 @@
-[Desktop Entry]
-Version=1.0
-Name=ChatGPT
-Exec=omarchy-launch-webapp https://chatgpt.com/
-Terminal=false
-Type=Application
-Icon=chatgpt
-StartupNotify=true