Implements the Groups & Routes admin page with a client-side RouteManager
component (add/delete sync routes via fetch), server-side groups page that
pre-fetches groups/routes from the API, and Next.js Route Handler proxies
for /api/routes (GET, POST) and /api/routes/[id] (DELETE). Adds a custom
jest environment that polyfills Node 18+ native fetch into the jsdom sandbox
so tests can use jest.spyOn(global, 'fetch').
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
