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: the status, one fixed line explaining it, and a link back to the webhook list, or to sign-in when nobody is signed in. Unknown paths reach it through the router's not-found handler, a refused form token through the CSRF middleware, and a panic through a recoverer each admin page route group installs first. Status codes are unchanged, and the page always sends Cache-Control: no-store.

If the error page fails to render, the answer is the same status in plain text; if it panics, the answer is a 500. The receiver, the healthcheck and /metrics keep their plain answers.

Model: opus-5-5

internal/handlers/delivery_replay.go

@@ -105,9 +105,7 @@ func (h *Handlers) HandleDeliveryReplay() 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
 		}
@@ -124,14 +122,14 @@ func (h *Handlers) replayDelivery(
 	webhook database.Webhook,
 ) {
 	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
 	}
@@ -173,7 +171,7 @@ func (h *Handlers) loadReplaySource(
 			&original, "id = ?", chi.URLParam(r, "deliveryID"),
 		).Error
 	if err != nil {
-		http.NotFound(w, r)
+		h.renderError(w, r, http.StatusNotFound)
 
 		return nil, false
 	}
@@ -195,7 +193,7 @@ func (h *Handlers) queueReplay(
 	)
 	if err != nil {
 		h.serverError(
-			w, "failed to count in-flight deliveries", err,
+			w, r, "failed to count in-flight deliveries", err,
 		)
 
 		return
@@ -212,7 +210,7 @@ func (h *Handlers) queueReplay(
 	err = webhookDB.
 		First(&event, "id = ?", original.EventID).Error
 	if err != nil {
-		h.serverError(w, "failed to load event for replay", err)
+		h.serverError(w, r, "failed to load event for replay", err)
 
 		return
 	}
@@ -222,7 +220,7 @@ func (h *Handlers) queueReplay(
 	)
 	if err != nil {
 		h.serverError(
-			w, "failed to create replay delivery", err,
+			w, r, "failed to create replay delivery", err,
 		)
 
 		return