Revise sync IO error wording to match blocking-route pattern
Headline: "Next.js encountered X during the initial render."
Explanation: "Without a prior data access, Next.js doesn't know
whether to prerender this value or compute it on each request."
Removes "fixed or fresh" terminology, "blocking navigation",
and "slower user experience" claims. Unifies the runtime and
regular message variants. Detection now uses docs URL patterns
instead of message text matching.
Made-with: Cursor
