Add ui.homepage field for redirecting to the app's homepage

[khandrew1]

Is your feature request related to a problem? Please describe.
There's no field for an app's actual homepage. ui.domain is a host-assigned sandbox origin, and the MCP server's own URL isn't necessarily the app's homepage (the app's public-facing site may be hosted separately from the MCP server). Hosts have no way to link users back to the app's real homepage.

Note that ChatGPT's Apps SDK already overloads _meta.ui.domain for something else: per their Build your MCP server docs, setting domain also controls whether the fullscreen "punch-out" button is shown, since ChatGPT renders the widget under <domain>.web-sandbox.oaiusercontent.com. That's a sandbox/rendering concern, separate from "where is the app's homepage."

Describe the solution you'd like
Add an optional homepage field to UIResourceMeta:

interface UIResourceMeta {
  // ...
  // The app's public homepage URL, which may differ from where the
  // MCP server itself is hosted.
  // Hosts MAY use this to let users navigate to the app's homepage.
  homepage?: string;
}