Add Node.js support via WebRTC polyfill injection

- Added WebRTCPolyfill interface for injecting WebRTC implementations
- Added wrtc option to RondevuOptions and RondevuConnectionParams
- Updated Rondevu and RondevuConnection to use injected APIs
- Added helpful error message when RTCPeerConnection is not available
- Updated README with Node.js usage examples
- Version bumped to 0.3.0

Fixes: RTCPeerConnection not defined error in Node.js

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/rondevu-client",
-  "version": "0.2.2",
+  "version": "0.3.0",
   "description": "TypeScript client for Rondevu peer signaling and discovery server",
   "type": "module",
   "main": "dist/index.js",