fix(security): stop embedding JWT tokens in SSO callback redirect URL (#4) (#14)

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

frontend/vitest.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+
+/// Vitest configuration for the Patch Manager UI.
+///
+/// - Uses jsdom for a browser-like environment (needed for MUI + React
+///   Testing Library).
+/// - The `react()` plugin is required for JSX in test files.
+/// - `globals: true` lets tests use `describe`, `it`, `expect` without
+///   imports (matches the existing frontend conventions).
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+  },
+})