Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/services/apiClientSetup.ts

@@ -0,0 +1,5 @@
+import { AxiosInstance } from 'axios';
+
+export function setupApiInterceptors(_client: AxiosInstance): void {
+  // Core version: no interceptors to add
+}