Simplify navi_ui instructions for small models ...

- mcp_servers.d/navi_ui.json: one-line tool call format, explicit component_name=card_grid, minimal schema, session_id auto-injected.

- navi/mcp/ui_server.py: flatten SERVER_INSTRUCTIONS to a strict JSON schema + numbered hard rules, remove mixed-format examples.

- navi/profiles/realtor/system_prompt.txt: replace YAML-ish example with exact JSON payload, numbered rules, clear session_id auto-injected note.

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

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>