fix(auth): backfill login form when injected defaults arrive after mount

[ggfto]

Problem

A deployment can inject gatewayUrl and gatewayToken into the page, and the
sign-in form still renders empty. Reloading never helps.

LoginGate seeds its local state from the auth store:

const gatewayUrlInit = useAuthStore((s) => s.gatewayUrl);
const [gatewayUrl, setGatewayUrl] = useState(gatewayUrlInit);

But that store is populated by an effect in <App>:

useEffect(() => {
  hydrate({ gatewayUrl, token: gatewayToken });
}, [hydrate, gatewayUrl, gatewayToken]);

React runs a parent's effects only after its children have mounted, so
LoginGate always reads the pre-hydration store — two empty strings. A plain
re-render never revisits a useState initializer, so the fields stay blank for
the lifetime of the page. The mount order is deterministic, which is why
reloading has no effect.

Fix

Backfill both fields once the defaults arrive, using functional updates so that
anything the user has already typed always wins:

useEffect(() => {
  setGatewayUrl((current) => current || gatewayUrlInit);
}, [gatewayUrlInit]);

18 lines in one component, plus tests.

What this deliberately does not change

The credential-restore policy stays exactly as it is. loadStoredCredentials()
is still never called on startup, which auth-store.ts explains is intentional:
a stale token or URL from a previous deployment would start a silent connect
attempt, leave the UI on "connecting", and block the user from submitting a
corrected value. This PR only fixes the prefill path, which is the one that is
already meant to run.

Tests

src/components/auth/LoginGate.test.tsx, two cases:

1. Backfills when defaults arrive after mount. Renders first, hydrates
   second — the same order the browser produces. This case fails without the
   change: expected '' to be 'ws://gateway.test/gateway-ws'.
2. Keeps values the user already typed, so a late default cannot clobber a
   correction in progress.

Full suite: 64 files, 546 tests passing. tsc --noEmit clean.

I left the repo's existing formatting untouched — running oxfmt with default
settings wanted to reflow unrelated lines in the same file, and that noise did
not belong in a fix this small.