Feature Request: Support Custom Server Configuration in aw-client-js

To support multi-user environments and remote server deployments, aw-client-js should allow explicit configuration of the server base_url and port.

Currently, the native Windows client achieves this via configuration files in the client's config folder, which enables users to connect to a server running on a different machine or custom port. However, aw-client-js lacks a similar mechanism, making it difficult for browser-based or custom clients to connect to non-default endpoints.

Adding support for custom server URLs in aw-client-js (e.g. via constructor options or environment variables) would bring it in line with the native clients and enable greater flexibility for advanced use cases like:
* Connecting to a centralized ActivityWatch server
* Running multiple clients across devices
* Developing third-party clients or extensions

Relevant code reference: [aw-client.ts L140](https://github.com/ActivityWatch/aw-client-js/blob/b2e02b3df9c398526619abc7c75507b0ab866ab3/src/aw-client.ts#L140)