feat(jade): create modular desktop environment

Rename xmonad/ to jade/ and restructure as a complete desktop environment
with hybrid NixOS and home-manager modules. This establishes jade as a
self-contained, gruvbox-themed DE that can eventually be extracted as a flake.

Changes:
- Create jade/ with NixOS module (WM, system packages, GTK theme)
- Create jade/home.nix for home-manager services (wezterm, picom, dunst)
- Move dmenu with gruvbox patches into jade/dmenu/
- Convert wezterm to pure Lua config with gruvbox-material colors
- Move xinitrc into jade/ directory
- Remove feh/scrot from global env (now in jade)
- Remove dmenu overlay from global overlays
- Simplify home-manager desktop features to just user apps

This follows the pattern of real DEs (GNOME, KDE) where system and user
configs are split across NixOS and home-manager modules.

nixos/common/global/env.nix

@@ -22,9 +22,6 @@
       # File Management
       fd ranger tree
 
-      # Graphics & Screenshots
-      feh scrot
-
       # Networking
       curl nmap sshfs wget