fix(layer): emit absolute sitemap URLs from configured site.url

[TerrorSquad]

Problem

/sitemap.xml gets its base URL from inferSiteURL(), which only reads env vars. A site.url set in nuxt.config is ignored, so it falls back to '' and every entry comes out relative:

<loc>/docs/getting-started/introduction</loc>

The sitemap spec requires absolute URLs, so search engines reject these. Nothing warns at build time.

Reproduction

Set site: { url: 'https://example.com' } in nuxt.config, no env vars, then nuxt build and fetch /sitemap.xml.

Fix

Expose the resolved site.url on runtimeConfig.public.site, the same way mcp.route is already exposed, and read it in the route. inferSiteURL() stays as the fallback so env-var deploys are unaffected.

Verified

Setup	Result
site.url in config	absolute URLs
env var only	unchanged
trailing slash in url	no double slash

pnpm lint and pnpm typecheck pass.

Note

pnpm install fails on a clean checkout: packageManager pins pnpm@11.13.0, which shipped a broken binary. Not related to this PR, happy to file separately.

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