Render admin page errors in the normal layout (closes #382)

Every 400, 403, 404 and 500 on an admin page now answers with an
error page in the normal layout: one fixed line for the status and a
link back to the webhook list, or to sign-in when nobody is signed
in. The router's handler for unknown paths, the CSRF middleware's
refusal and a panic in an admin page route group use the same page;
each such group has its own recoverer and error reporting for that.
The page always sends Cache-Control: no-store. Status codes are
unchanged. The receiver, the healthcheck and /metrics keep their
plain answers. If the error page cannot render, or panics, the answer
is the same status in plain text.

Model: opus-5-5

internal/handlers/event_resubmit.go

@@ -99,7 +99,7 @@ func (h *Handlers) HandleEventResubmit() http.HandlerFunc {
 		// middleware, which runs before CSRF parses the form.
 		err := r.ParseForm()
 		if err != nil {
-			http.Error(w, "Bad request", http.StatusBadRequest)
+			h.renderError(w, r, http.StatusBadRequest)
 
 			return
 		}
@@ -120,20 +120,20 @@ func (h *Handlers) resubmitEvent(
 	// alphabet rather than from the request.
 	eventID, err := uuid.Parse(chi.URLParam(r, "eventID"))
 	if err != nil {
-		http.NotFound(w, r)
+		h.renderError(w, r, http.StatusNotFound)
 
 		return
 	}
 
 	if !h.dbMgr.DBExists(webhook.ID) {
-		http.NotFound(w, r)
+		h.renderError(w, r, http.StatusNotFound)
 
 		return
 	}
 
 	webhookDB, err := h.dbMgr.GetDB(webhook.ID)
 	if err != nil {
-		h.serverError(w, "failed to get webhook database", err)
+		h.serverError(w, r, "failed to get webhook database", err)
 
 		return
 	}
@@ -147,7 +147,7 @@ func (h *Handlers) resubmitEvent(
 		webhookDB, webhook.ID, eventID.String(),
 	)
 	if err != nil {
-		h.serverError(w, "failed to load event to resubmit", err)
+		h.serverError(w, r, "failed to load event to resubmit", err)
 
 		return
 	}
@@ -155,7 +155,7 @@ func (h *Handlers) resubmitEvent(
 	// A miss is a 404 whether the event was reaped, belongs to
 	// another webhook, or never existed.
 	if !found {
-		http.NotFound(w, r)
+		h.renderError(w, r, http.StatusNotFound)
 
 		return
 	}
@@ -207,7 +207,7 @@ func (h *Handlers) queueResubmit(
 	// inactive one is skipped rather than refused.
 	targets, err := h.loadActiveTargets(webhook.ID)
 	if err != nil {
-		h.serverError(w, "failed to query targets", err)
+		h.serverError(w, r, "failed to query targets", err)
 
 		return
 	}
@@ -225,7 +225,7 @@ func (h *Handlers) queueResubmit(
 		targets,
 	)
 	if err != nil {
-		h.serverError(w, "failed to store resubmitted event", err)
+		h.serverError(w, r, "failed to store resubmitted event", err)
 
 		return
 	}