Support canonical URL

ref https://github.com/nuxt/nuxt/issues/24813

Today, we expose useRequestURL() utility to access the current request URL however it can be tricky in two situations:

• In SSG, we cannot access the desired URL
• In runtime, sometimes, we need a canonical URL different from the incoming request
• In cached event handlers, we don't have access to incoming headers without opting more headers to varies (which can also lead to cache leaks)
• When we require a full URL for generating content there is no one standard way to depend on
• With fetch(/path) we need a default host (instead of assuming localhost)

Proposal:

• Support app.canonicalURL in runtimeConfig for env support + app config for dynamic behavior support (prefer app)
• Use canonical URL as default fallback for useRequestURL()
• Introduce new useCanonicalURL() (suggested by @Atinux) that falls back to useRequestURL() if the user hasn't configured a canonical URL and fails for SSG if not provided

Start by reading the existing useRequestURL() utility and the runtimeConfig and app config handling referenced in the proposal. Trace how URLs are resolved in SSG, runtime requests, cached event handlers, and fetch(/path). Done means app.canonicalURL and a useCanonicalURL() fallback are supported, with the stated SSG failure behavior when no canonical URL is configured.