feat: add COSMIC desktop, greetd DM, and transparent logo to root-overlay

- Add COSMIC desktop subgroup to online netinstall (galaxy repo)
- Add greetd as display manager option in packagechooser_dm.conf
  (required by COSMIC's cosmic-greeter)
- Replace non-transparent 900x650 logo with transparent removebg version
  at 128x92 (128x128) and 256x185 (256x256) in root-overlay icon theme paths
- Update docs to reflect COSMIC availability and greetd option

docs/desktop-environments.md

@@ -73,6 +73,15 @@ Available desktops in online mode, sourced from Artix repos.
 | **Packages** | `hyprland` |
 | **Session** | `hyprland.desktop` |
 
+## COSMIC
+
+| | |
+|---|---|
+| **Repo** | galaxy |
+| **Group** | `cosmic` |
+| **Session** | `cosmic.desktop` |
+| **Notes** | Rust-based, Wayland-native. Uses greetd + cosmic-greeter (not SDDM). Alpha quality, not for production. Works with dinit, s6, runit; OpenRC may have 100% CPU bugs |
+
 ## No Desktop
 
 Installs the base system only (TTY/login). Useful for servers or custom setups.