Let `npm run dev` reach fake-engine in a plain browser

The frontend can only learn the engine's address from `window.ducklab`, which the Wails shell injects — so a frontend contributor must build and install the whole desktop to see any change live, while `cmd/fake-engine` (built precisely so the frontend can be exercised with no model, no GPU and no key) is reachable only through the playwright fixtures. **Done means:** in dev builds, when `window.ducklab` is absent, the client reads the engine address and token from query params (or Vite env), so the loop is: ```bash go run ./cmd/fake-engine --port 8899 --scenario pair cd frontend && npm run dev # open http://localhost:5173/?engine=http://127.0.0.1:8899&token=fake-token ``` Where to look: `frontend/src/app/App.tsx` (the `window.ducklab` read), `frontend/e2e/fixtures.ts` (how playwright injects it today). Tracked on the project's own board as B-108.