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/apiClient.ts

@@ -0,0 +1,25 @@
+import axios from 'axios';
+import { handleHttpError } from '@app/services/httpErrorHandler';
+import { setupApiInterceptors } from '@app/services/apiClientSetup';
+
+// Create axios instance with default config
+const apiClient = axios.create({
+  baseURL: import.meta.env.VITE_API_BASE_URL || '/',
+  responseType: 'json',
+});
+
+// Setup interceptors (core does nothing, proprietary adds JWT auth)
+setupApiInterceptors(apiClient);
+
+// ---------- Install error interceptor ----------
+apiClient.interceptors.response.use(
+  (response) => response,
+  async (error) => {
+    await handleHttpError(error); // Handle error (shows toast unless suppressed)
+    return Promise.reject(error);
+  }
+);
+
+
+// ---------- Exports ----------
+export default apiClient;