Function LoadClientConfig

Resolves the MJAPI GraphQL endpoint and system API key from env:

• MJAPI_URL overrides everything; otherwise http://localhost:{GRAPHQL_PORT}{GRAPHQL_ROOT_PATH}
• MJAPI_WS_URL overrides the subscription endpoint; otherwise derived from Url (http→ws, https→wss)
• MJ_API_KEY is the system API key MJServer accepts via the x-mj-api-key header