fix(frontend): fix custom mutator of orval (#10269)

This pull request includes updates to the environment configuration and
API mutator logic in the `autogpt_platform/frontend` directory. The
changes aim to improve flexibility by introducing dynamic base URLs
through environment variables.

Environment configuration updates:

*
[`autogpt_platform/frontend/.env.example`](diffhunk://#diff-72012a00359825421736dc064be74187011cb5b0462bea1ed3a3c5ca80bb3117R2):
Added `NEXT_PUBLIC_FRONTEND_BASE_URL` to define the base URL for the
frontend dynamically.

API mutator logic updates:

*
[`autogpt_platform/frontend/src/app/api/mutators/custom-mutator.ts`](diffhunk://#diff-28c5af33c7bd0ecddc1793aa6a27bfd5b4f979b62c29990538aceea3320d8be9L1-R1):
Updated `BASE_URL` to use the `NEXT_PUBLIC_FRONTEND_BASE_URL`
environment variable, enabling dynamic configuration of the API proxy
URL.

### Checklist
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
   - [x] Tested manually and everything is working perfectly

autogpt_platform/frontend/.env.example

@@ -1,4 +1,5 @@
 FRONTEND_BASE_URL=http://localhost:3000
+NEXT_PUBLIC_FRONTEND_BASE_URL=http://localhost:3000
 
 NEXT_PUBLIC_AUTH_CALLBACK_URL=http://localhost:8006/auth/callback
 NEXT_PUBLIC_AGPT_SERVER_URL=http://localhost:8006/api

autogpt_platform/frontend/src/app/api/mutators/custom-mutator.ts

@@ -1,4 +1,4 @@
-const BASE_URL = "/api/proxy"; // Sending request via nextjs Server
+const BASE_URL = `${process.env.NEXT_PUBLIC_FRONTEND_BASE_URL}/api/proxy`; // Sending request via nextjs Server
 
 const getBody = <T>(c: Response | Request): Promise<T> => {
   const contentType = c.headers.get("content-type");