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/tsconfig.json

@@ -0,0 +1,18 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "baseUrl": "../../",
+    "paths": {
+      "@app/*": [
+        "src/core/*"
+      ]
+    }
+  },
+  "include": [
+    "../global.d.ts",
+    "../*.js",
+    "../*.ts",
+    "../*.tsx",
+    "."
+  ]
+}