feat: add custom brand colours to emails (#3005)

packages/email/preview/app/routes/_index.tsx

@@ -0,0 +1,13 @@
+import { redirect } from 'react-router';
+
+import { templates } from '../lib/templates';
+
+/**
+ * The index has no UI of its own — redirect to the first template so the
+ * preview always opens on something.
+ */
+export const loader = () => {
+  const firstSlug = Object.keys(templates)[0];
+
+  return redirect(`/${firstSlug}`);
+};