Targets section: one Add, then a type and Next, then its fields (closes #370)

The webhook page's targets section lists only its targets until Add is
clicked. Add shows a choice of target type with Next; Next shows only
that type's fields, with Save and Cancel. The `database` and `log`
types have no URL field, and the `slack` form gains max retries.

A refused target now shows the webhook page again with the form open on
its type, the values entered and the reason, instead of a bare text
page. Target validation returns that message rather than writing the
response; newTarget validates a whole new target for reuse by the
new-webhook page. The edit page still answers a refusal in plain text.

Model: opus-5-5

internal/handlers/target_edit.go

@@ -127,11 +127,12 @@ func (h *Handlers) applyTargetEdit(
 		return
 	}
 
-	configJSON, err := h.buildTargetConfig(
-		w, r, target.Type, targetFormInputFrom(r),
+	configJSON, errMsg := h.buildTargetConfig(
+		r.Context(), target.Type, targetFormInputFrom(r),
 	)
-	if err != nil {
-		// buildTargetConfig has already written the response.
+	if errMsg != "" {
+		http.Error(w, errMsg, http.StatusBadRequest)
+
 		return
 	}
 
@@ -161,7 +162,7 @@ func (h *Handlers) applyTargetEdit(
 	// A new name renames the archive file before it is saved (see
 	// delivery.Engine.Rename). If either step fails, it goes back to
 	// the name that is still stored.
-	err = h.renameTargetArchive(target, webhook.Name, oldName, name)
+	err := h.renameTargetArchive(target, webhook.Name, oldName, name)
 	if err == nil {
 		err = h.db.DB().Save(target).Error
 	}