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>

lyrathorpe/home/desktop.nix

@@ -2,7 +2,13 @@
 # Imported only on hosts that run Sway (MBP, T400, Mac Pro); never pulled onto
 # the headless WSL host. Login (and the Sway session launch) is handled by the
 # greetd/ReGreet greeter -- see ../swaywm.nix -- so there is no tty1 autostart.
-{ pkgs, config, ... }:
+{
+  pkgs,
+  config,
+  inputs,
+  username,
+  ...
+}:
 {
   imports = [
     ./sway.nix
@@ -57,4 +63,31 @@
     name = "Adwaita";
     size = 24;
   };
+
+  # Firefox is themed at the browser level (it does not follow the GTK theme).
+  # The system installs the binary (programs.firefox in ../user.nix); here
+  # home-manager owns only the profile, hence package = null. Apply the
+  # Catppuccin Mocha theme add-on (only the mauve accent is packaged upstream;
+  # the rest of the desktop uses blue) and make content + UI dark.
+  programs.firefox = {
+    enable = true;
+    package = null;
+    profiles.${username} = {
+      id = 0;
+      isDefault = true;
+      extensions = {
+        force = true;
+        packages = [
+          inputs.firefox-addons.packages.${pkgs.system}.catppuccin-mocha-mauve
+        ];
+      };
+      settings = {
+        # Enable bundled add-ons automatically so the theme applies on first run.
+        "extensions.autoDisableScopes" = 0;
+        # Dark chrome + dark page content.
+        "ui.systemUsesDarkTheme" = 1;
+        "layout.css.prefers-color-scheme.content-override" = 0;
+      };
+    };
+  };
 }