Browser example: @rdcp.dev/client usage (standalone demo)

Add a minimal browser-based example demonstrating @rdcp.dev/client usage (or a small wrapper) against a demo server. Optional follow-up to #75.\n\nScope\n- Basic HTML+JS page (no build step) or Vite example\n- Discovery, status, control, health\n- Auth discussion (bearer only; no secrets in browser)\n\nLinkage\n- Follow-up to #75