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/routes/authShared/GuestSignInButton.tsx

@@ -0,0 +1,24 @@
+import React from 'react'
+import '@app/routes/authShared/auth.css'
+import '@app/routes/authShared/saas-auth.css'
+
+interface GuestSignInButtonProps {
+  label: string
+  onClick: () => void
+  disabled?: boolean
+}
+
+export default function GuestSignInButton({ label, onClick, disabled }: GuestSignInButtonProps) {
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      disabled={disabled}
+      className="w-full px-4 py-[0.75rem] rounded-[0.625rem] text-base font-semibold mb-2 cursor-pointer disabled:opacity-50 disabled:cursor-not-allowed auth-guest-button"
+    >
+      {label}
+    </button>
+  )
+}
+
+