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

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;
}

Locate the TypeScript UIResourceMeta interface and review nearby metadata definitions and validation or type tests. Add the optional homepage URL field with the documented semantics, then verify that existing metadata remains valid and the new field is accepted.