Add configs/ folder: Firefox prefs via autoconfig, Gruvbox fastfetch

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

modules/firefox.nix

@@ -0,0 +1,11 @@
+{ config, pkgs, ... }:
+
+{
+  programs.firefox = {
+    enable = true;
+    # Not locked: values are reapplied on every Firefox start, but can be
+    # changed in the UI for the current session. (Read as text: the module
+    # doesn't copy autoConfigFiles paths into the store.)
+    autoConfig = builtins.readFile ../configs/firefox/prefs.js;
+  };
+}