feat: add SSE subscription for live market trade events

[opalo1992]

Summary

This PR introduces a new method subscribeMarketTradesEvents that enables
real-time market trade event streaming using Server-Sent Events (SSE).

Motivation

The current client only provides getMarketTradesEvents, which requires
polling. SSE provides lower latency, reduced server load, and improved UX for
trading bots and real-time dashboards.

Implementation details

• Added subscribeMarketTradesEvents method with browser/Node support.
• Implemented SSE streaming with graceful error handling.
• Added Node polyfill eventsource.
• Added import of isBrowser from browser-or-node.
• No breaking changes.

Testing

Manual testing via browser and Node environments confirms correct message
streaming and reconnection behavior.

---

Note

Introduce subscribeMarketTradesEvents for real-time market trades via SSE with browser/Node support; add eventsource dependency.

• Client (src/client.ts):
  • Add subscribeMarketTradesEvents(conditionID, onMessage, onError?) to stream market trade events via SSE and return an unsubscribe function.
  • Support both environments using isBrowser and Node fallback with require("eventsource"); include geo_block_token in URL.
  • Minor setup: import isBrowser, createRequire; define require via createRequire.
• Dependencies:
  • Add eventsource runtime dependency in package.json.

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