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/core/components/home/HomePageExtensions.tsx

@@ -0,0 +1,7 @@
+/**
+ * Core stub for HomePage extensions.
+ */
+
+export function HomePageExtensions() {
+  return null;
+}