Introduce a standalone Traefik Manager mode and SPA: new internal/traefikmanager package (config, helpers, files, settings, runtime, handlers, server, types) with tests and supporting testdata; add a new React/TypeScript SPA under traefik-ui/ (components, pages, build config, tests). Update build & packaging: modify Dockerfile to build the traefik-ui app and include it in the final image, add new Makefile targets for building/running both UIs and modes, and add a traefik-manager service to docker-compose (with MODE env). Add runtime config loading and README docs for the new MODE, plus .gitignore entries for traefik-ui artifacts. Small updates to main.go and added traefik_mode.go.
