Broken nav link fails the build: content/nav.config.ts entries are validated at build time. A href that doesn't match /, /dashboard, /dashboard/editor, or a file in content/docs/ fails the build rather than shipping a 404. Fix the nav entry or add the missing .mdx file.
Assets 404 under a subpath: usually means BASE_PATH was set for the app but not consistently across assetPrefix; both are derived from the same env var in next.config.mjs, so re-check the variable is set at build time, not runtime.
The top bar and page titles show MyProduct: PRODUCT_NAME wasn't set at build time. Set it in .env.local for local builds, or in your hosting provider's dashboard for production, and rebuild.
Canonical URLs still show example.com: SITE_URL wasn't set at build time. Re-run the build with it exported.