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/proprietary/services/apiClientSetup.ts

@@ -0,0 +1,29 @@
+import { AxiosInstance } from 'axios';
+
+function getJwtTokenFromStorage(): string | null {
+  try {
+    return localStorage.getItem('stirling_jwt');
+  } catch (error) {
+    console.error('[API Client] Failed to read JWT from localStorage:', error);
+    return null;
+  }
+}
+
+export function setupApiInterceptors(client: AxiosInstance): void {
+  // Install request interceptor to add JWT token
+  client.interceptors.request.use(
+    (config) => {
+      const jwtToken = getJwtTokenFromStorage();
+
+      if (jwtToken && !config.headers.Authorization) {
+        config.headers.Authorization = `Bearer ${jwtToken}`;
+        console.debug('[API Client] Added JWT token from localStorage to Authorization header');
+      }
+
+      return config;
+    },
+    (error) => {
+      return Promise.reject(error);
+    }
+  );
+}