* feat(mcp-server): add DRAWIO_BASE_URL env for private deployments
* Fix postMessage origin check and URL normalization
- Add getOrigin() function to extract scheme+host+port from DRAWIO_BASE_URL
- Use DRAWIO_ORIGIN for postMessage security check instead of full URL
- Add normalizeUrl() to remove trailing slash and avoid double slashes
- This fixes issues when users configure DRAWIO_BASE_URL with trailing slash or path
