Replaces URL-embedded JWT tokens with a single-use, 60-second handoff code that the SPA exchanges via server-to-server POST. The URL now contains only `?handoff=<code>` — no tokens are placed in the browser history, proxy access logs, or Referer header.
Backend: new SsoHandoff store (DashMap, 60s TTL, atomic DashMap::remove for single-use), POST /api/v1/auth/sso/handoff endpoint, 7 new tests.
Frontend: SsoCallbackPage rewritten to use useSearchParams + POST exchange, with history.replaceState to clear the handoff code from the address bar. Switched from window.location.search to useSearchParams() for test compatibility. New Vitest infrastructure (vitest, @testing-library/react, jsdom) and 6 new tests.
CI fix in ccba9e3: cargo fmt --all and added searchParams to useEffect dep array to satisfy CI's Rust Format and Frontend Lint checks.
Refs: closes#4