Show the Name servers check as it happens, and keep its results

The zone's Name servers tab no longer waits for the other servers before
showing: it lists each query as a dig command line with a waiting
counter, fills in the answers as they arrive, then shows the result with
the queries under "Queries sent". Results (serials and delegation) are
kept for 10 minutes so revisiting the tab is instant; "Check again" asks
anew, and a check keeps running when the tab is left.

GET /zones/{zone}/serials?stream=1 streams the queries as
newline-delimited JSON (nscheck.Checker.Watch); without it the endpoint
is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/server.go

@@ -384,6 +384,10 @@ func (w *statusWriter) WriteHeader(code int) {
 	w.ResponseWriter.WriteHeader(code)
 }
 
+// Unwrap lets http.ResponseController reach the connection (to flush a
+// streamed response).
+func (w *statusWriter) Unwrap() http.ResponseWriter { return w.ResponseWriter }
+
 func (w *statusWriter) Write(b []byte) (int, error) {
 	if w.status == 0 {
 		w.status = http.StatusOK