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/tsconfig.saas.vite.json

@@ -0,0 +1,22 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "paths": {
+      "@app/*": [
+        "src/saas/*",
+        "src/proprietary/*",
+        "src/core/*"
+      ],
+      "@proprietary/*": ["src/proprietary/*"],
+      "@core/*": ["src/core/*"]
+    }
+  },
+  "exclude": [
+    "src/core/**/*.test.ts*",
+    "src/core/**/*.spec.ts*",
+    "src/proprietary/**/*.test.ts*",
+    "src/proprietary/**/*.spec.ts*",
+    "src/desktop",
+    "node_modules"
+  ]
+}