UI: Add support for calling API endpoints on remote llama-server

[niutech]

Overview

This PR allows the web UI to call API endpoints on remote llama-server, by optionally setting API_BASE and API_KEY settings in the UI. Implements #22531.

Additional information

In order to test it:

1. Run public instance of llama-server on https://example.org (optionally with --api-key)
2. Go to tools/ui and run npm run build && npm run preview
3. Open http://localhost:4173/#/settings, set API Base to https://example.org and optionally your API Key, then save settings
4. Chat with a remote llama-server

Screenshot

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO