fix(notifications): a notice's button is said when shown, like its text

An action label was translated when the notice was raised, so after a language switch a live
toast or center row read in one language with its button in the other. The access-requests nudge
no longer re-issues on a switch for it. New tests: the button and a server refusal follow a
switch.

frontend/src/modules/channels/useChannelActions.tsx

@@ -74,7 +74,7 @@ export function useChannelActions({
         level: "success",
         message: msg("channels.notify.hidden", { name: nameOf(c) }),
         action: {
-          label: t("common.undo"),
+          label: msg("common.undo"),
           onClick: () => patch.mutate({ id: c.id, body: { hidden: false } }),
         },
       });