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>

home-manager/fred.nix

@@ -1,5 +1,5 @@
 # settings/fred.nix
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, inputs, ... }:
 {
   # Define the state version for Home Manager
   home.stateVersion = "25.11";
@@ -20,7 +20,7 @@
     font-family = FiraMono Nerd Font
     font-size = 11
     font-thicken = true
-    theme = dark:Catppuccin Mocha,light:Catppuccin Latte
+    theme = wallpaper
     window-padding-x = 10
     window-padding-y = 10
     window-padding-balance = true
@@ -31,4 +31,19 @@
     cursor-style-blink = true
   '';
 
+  # Matugen config — templates for wallpaper-based color generation
+  home.file.".config/matugen/config.toml".text = ''
+    [config]
+    reload_apps = true
+    reload_apps_list = { ghostty = "" }
+
+    [templates.ghostty]
+    input_path = "${inputs.self}/templates/ghostty-colors"
+    output_path = "~/.config/ghostty/themes/wallpaper"
+
+    [templates.gtk4]
+    input_path = "${inputs.self}/templates/gtk4-colors.css"
+    output_path = "~/.config/gtk-4.0/colors.css"
+  '';
+
 }