Add SaaS frontend code (#5879)

# Description of Changes
Adds the code for the SaaS frontend as proprietary code to the OSS repo.
This version of the code is adapted from 22/1/2026, which was the last
SaaS version based on the 'V2' design. This will move us closer to being
able to have the OSS products understand whether the user has a SaaS
account, and provide the correct UI in those cases.

frontend/src/saas/hooks/useConfigButtonIcon.tsx

@@ -0,0 +1,7 @@
+import { Avatar } from '@mantine/core';
+import { useAuth } from '@app/auth/UseSession';
+
+export function useConfigButtonIcon(): React.ReactNode {
+  const { profilePictureUrl } = useAuth();
+  return profilePictureUrl ? <Avatar src={profilePictureUrl} radius="xl" size={24} /> : null;
+}