feat: add thunderbird, move ametrine to aux

modules/nixos/apps/thunderbird/default.nix

@@ -0,0 +1,18 @@
+{ config, pkgs, lib, ... }:
+let
+  cfg = config.plusultra.apps.thunderbird;
+in
+{
+  options.plusultra.apps.thunderbird = {
+    enable = lib.mkEnableOption "Thunderbird";
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.thunderbird;
+      description = "Thunderbird package to use";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages = [ cfg.package ];
+  };
+}

systems/x86_64-linux/bismuth/default.nix

@@ -72,6 +72,7 @@ with lib.${namespace};
       ubports-installer = enabled;
       steamtinkerlaunch = enabled;
       r2modman = enabled;
+      thunderbird = enabled;
     };
 
     services = {