* fix(404): align default not-found copy with Next.js
The Pages Router previously emitted `<h1>404 - Page not found</h1>` and the
App Router emitted plain-text `This page could not be found` (no period) when
no user-defined 404 boundary was present. Next.js renders
`This page could not be found.` (with trailing period) for both routers.
Closes#1454.
* fix(404): address bot review — charset attribute and h1 line-height
- Use HTML attribute name 'charset' (not the JSX 'charSet') in the raw HTML template.
- Drop stray 'line-height:49px' on the Pages Router h1 — Next.js's _error.tsx inherits 48px from the parent div.
James Anderson