`gonext generate page <name>` — Next.js route with a typed API call

Scaffolds a Next.js route in the generated frontend together with a matching call through the typed OpenAPI client that #69 introduced, so a new page starts already wired to the backend contract instead of with a hand-written fetch.

Split out of #15.

Start at the `gonext generate page ` entry point and trace how the typed OpenAPI client introduced by #69 is exposed to generated frontend pages. Done means the command scaffolds a Next.js route with a matching typed client call, rather than a hand-written fetch.

Written by the indexing model from the issue text.