feat(firefox): theme with the Catppuccin Mocha add-on

Firefox draws its own chrome and ignores the GTK theme, so theme it at the
browser level. Add the rycee firefox-addons flake input and, in the
home-manager desktop layer, manage the Firefox profile (package = null --
the system programs.firefox in user.nix still provides the binary):

- install the Catppuccin Mocha theme add-on (catppuccin-mocha-mauve; only
  the mauve accent is packaged upstream, so it differs slightly from the
  blue accent used elsewhere),
- autoDisableScopes = 0 so it applies on first launch,
- ui.systemUsesDarkTheme + prefers-color-scheme override for dark chrome
  and page content.

Verified the XPI fetches, user.js renders the prefs, finalPackage is null
(no duplicate Firefox), all Sway hosts eval, and EDaaS is unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

flake.nix

@@ -23,6 +23,11 @@
     # Provides mkFlake: the systems/perSystem scaffolding used below.
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+    # Declarative Firefox add-ons (e.g. the Catppuccin theme); see lyrathorpe/user.nix.
+    firefox-addons = {
+      url = "gitlab:rycee/nur-expressions?dir=pkgs/firefox-addons";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =