Allow login to SaaS for desktop instead of offline mode (#4941)

# Description of Changes
Makes the desktop options to sign in with your Stirling account, or sign
into self-hosted:

<img width="608" height="456" alt="image"
src="https://github.com/user-attachments/assets/a49988ab-db3f-4333-b242-790aee5c07c6"
/>

The first option still runs everything locally, just enforces that
you've signed in for now. Future work will enable sending operations
that can't be run locally to the server.

frontend/src/desktop/constants/connection.ts

@@ -2,4 +2,11 @@
  * Connection-related constants for desktop app
  */
 
-export const STIRLING_SAAS_URL = 'https://stirling.com/app';
+// SaaS server URL from environment variable (required)
+// The SaaS authentication server
+// Will throw error if VITE_SAAS_SERVER_URL is not set
+if (!import.meta.env.VITE_SAAS_SERVER_URL) {
+  throw new Error('VITE_SAAS_SERVER_URL environment variable is required');
+}
+
+export const STIRLING_SAAS_URL = import.meta.env.VITE_SAAS_SERVER_URL;