add /chat route — in-app chat against the gateway

Uses the user's JWT against /v1/models + /v1/chat/completions (the
gateway's /v1 routes already accept JWT — no API key needed for the
in-app chat path; API keys remain for external SDK usage).

Single-turn UX for now, no streaming. Model picker reads /v1/models.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes.ts

@@ -6,4 +6,5 @@ export default [
   route('verify', 'routes/verify.tsx'),
   route('login', 'routes/login.tsx'),
   route('account', 'routes/account.tsx'),
+  route('chat', 'routes/chat.tsx'),
 ] satisfies RouteConfig;