Add internal navi_ui MCP server for structured UI components ...

- Add navi/mcp/ui_server.py: FastMCP streamable_http server on port 8001
  exposing render_component(component_name, payload, session_id).
- Start server in main lifespan before container creation so McpManager can
  connect; wire orchestrator once container is ready; clean up on shutdown.
- Add env settings NAVI_UI_MCP_ENABLED/HOST/PORT.
- Add mcp_servers.d/navi_ui.json config with the 'ui' tool group.
- Frontend: dispatch ui_component websocket event, store in chat.js, render
  placeholder UiComponentCard inside AssistantMessage.vue.
- Unit tests for ui_server tool and chat.onUiComponent.

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