Add wallpaper-based color theming with matugen

- Matugen templates for Ghostty theme and GTK4 colors
- Ghostty uses generated wallpaper theme instead of Catppuccin
- GTK4 CSS imports generated color overrides
- Update alias runs matugen after switch to regenerate colors
- Add wallpaper fish function to change wallpaper + regen colors

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

settings/gnome.nix

@@ -67,6 +67,7 @@
       # Minimal titlebars — hide window buttons and shrink headerbar
       home.file.".config/gtk-4.0/gtk.css".force = true;
       home.file.".config/gtk-4.0/gtk.css".text = ''
+        @import url("colors.css");
         headerbar {
           min-height: 0;
           padding: 0;