v0.7.0

feat: built-in MCP server for AI agent integration

- Rust axum HTTP server implements MCP Streamable HTTP protocol
- McpBuffer accumulates receive text, exposed via resources/tools
- Tools: get_receive_content, get_port_status, send_serial_data, clear_receive
- Settings: enable/disable, configurable port (default 9876)
- Titlebar toggle button with status indicator
- Help menu → MCP prompt dialog with copyable config