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/playlists/Playlists.tsx

@@ -547,7 +547,7 @@ function PlaylistDetail() {
       level: "success",
       message: msg("playlists.removedToast", { title: raw(video.title ?? "") }),
       action: {
-        label: t("common.undo"),
+        label: msg("common.undo"),
         // ONE request that puts it back at its index (the server renumbers around it): an append
         // plus a whole-list reorder would land it at the end whenever the second call failed. The
         // index is worked out against the list as it is NOW, fetched fresh.