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/components/shared/PrivateContent.tsx

@@ -0,0 +1,30 @@
+import React from 'react';
+
+interface PrivateContentProps extends React.HTMLAttributes<HTMLSpanElement> {
+  children: React.ReactNode;
+}
+
+/**
+ * SaaS override of the OSS PrivateContent wrapper.
+ * Adds both the PostHog no-capture class and the Userback opt-out class
+ * while keeping the same API and layout behavior (display: contents).
+ */
+export const PrivateContent: React.FC<PrivateContentProps> = ({
+  children,
+  className = '',
+  style,
+  ...props
+}) => {
+  const baseClass = 'ph-no-capture userback-block';
+  const combinedClassName = className ? `${baseClass} ${className}` : baseClass;
+  const combinedStyle = {
+    display: 'contents' as const,
+    ...style,
+  };
+
+  return (
+    <span className={combinedClassName} style={combinedStyle} {...props}>
+      {children}
+    </span>
+  );
+};