Extend wallpaper theming to Zen, VSCodium, and GNOME accents

- Zen browser: generates userChrome.css with palette colors
- VSCodium: merges workbench.colorCustomizations into settings.json
- GNOME: maps palette hue to closest accent color preset
- Add jq to GNOME packages for JSON merging

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

settings/gnome.nix

@@ -25,6 +25,7 @@
       papirus-icon-theme
       adw-gtk3
       matugen
+      jq
     ];
 
     # Set cursor theme