feat(surf): branded HTML error pages for browser navigations

A browser navigation (Sec-Fetch-Mode navigate, or GET/HEAD without it; no
X-Requested-With; Accept naming text/html above every JSON type; not /api,
/_<name> or the admin prefix) that hits a framework-generated 404, 405 or
500 gets a self-contained SummerCMS page: unmatched routes and wrong
methods at the ServeMux, failed Where/WhereIn constraints, overlap-family
misses, the unpublished uploads route and recovered panics. Every other
request keeps the exact stdlib text, wire's opaque JSON 500 or the bare
raw-group 500.

Details (method, path, X-Request-Id) only with app.debug or outside
production. Applications replace the page with a published
surf.ErrorPageRenderer; http.error_pages.enabled false turns it off.

modules/surf/params.go

@@ -78,14 +78,16 @@ func (c Constraint) match(value string) bool {
 	return false
 }
 
-func constrain(next http.Handler, cs []Constraint) http.Handler {
+// constrain answers 404 (the error page for a browser navigation, pages
+// permitting) when a Where/WhereIn constraint fails.
+func constrain(pages *errorPages, next http.Handler, cs []Constraint) http.Handler {
 	if len(cs) == 0 {
 		return next
 	}
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		for _, c := range cs {
 			if !c.match(r.PathValue(c.param)) {
-				http.NotFound(w, r)
+				pages.notFound(w, r)
 				return
 			}
 		}