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/constants/app.ts

@@ -0,0 +1,15 @@
+// Re-export all constants from core
+export * from '@core/constants/app';
+
+// SaaS-specific overrides
+// Get base URL with fallback (for use outside React components)
+export const getBaseUrl = (): string => {
+  // Try to get from window object if set by app config
+  const baseUrl = (window as any).__STIRLING_PDF_BASE_URL__ || window.location.origin;
+  return baseUrl;
+};
+
+// Helper to set base URL (to be called when app config loads)
+export const setBaseUrl = (url: string): void => {
+  (window as any).__STIRLING_PDF_BASE_URL__ = url;
+};