fix: series removal sent its status line twice

adminSeriesRemove answered the list surface with two h.render calls -- the
row fragment and the out-of-band heading -- and h.render writes a status
line each time, so every removal logged "superfluous
response.WriteHeader call". The heading is an append to a response
already committed, so it now executes straight onto w, the way
writeChromeOOB already does it.

The regression test runs the router under a real server with a captured
ErrorLog: a ResponseRecorder never sees this warning, which is why the
existing removal test did not catch it.

backend/internal/web/admin_series.go

@@ -505,10 +505,13 @@ func (h *Handler) adminSeriesRemove(w http.ResponseWriter, r *http.Request) {
 		band = 1
 	}
 	h.render(w, http.StatusOK, "series-row", seriesRow(a, band, time.Now()))
+	// The heading is an out-of-band append to a response whose status line has
+	// already gone out with the row, so it is executed straight onto w —
+	// h.render would send a second WriteHeader.
 	if head, err := h.seriesListHeadView(r); err != nil {
 		log.Printf("series remove %s: %v", site+":"+seriesID, err)
-	} else {
-		h.render(w, http.StatusOK, "series-list-head", head)
+	} else if err := h.tmpl.ExecuteTemplate(w, "series-list-head", head); err != nil {
+		log.Printf("series remove %s: render series-list-head oob: %v", site+":"+seriesID, err)
 	}
 }