feat: Client browser

[Pederzh]

Description

This PR adds browser support for the MCP client library and includes a comprehensive React example demonstrating its usage. The changes enable developers to use MCP in browser environments with WebSocket and HTTP/SSE transports, along with OAuth 2.0 authentication support.

Key additions:

• Browser-compatible MCP client (src/browser.ts) - Entry point for browser usage
• Refactored client architecture - Extracted base client logic to src/client/base.ts for code reuse
• OAuth 2.0 helper (src/oauth-helper.ts) - Full OAuth flow implementation with dynamic client registration
• React example - Complete working example in examples/react/ with Vite configuration and browser polyfills
• Browser-safe logging - Updated logging to fall back to console in browser environments

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• The title of my pull request follows the conventional commits standard
• Changes have been documented in the README/documentation (if applicable)