feat(audit): undo button for merges, hide manual games

The audit log shows Undo on a merge row for 30 days, with a confirm
dialog and clear errors. The details page follows a merged key to the
kept game. Hide and Show now work for manual games too. de/en texts.

src/utils/links.js

@@ -47,3 +47,9 @@ export const fetchAuditLog = ({action, page, size}) => {
     if (action) params.set('action', action);
     return apiFetch(`${base()}/audit-log?${params}`);
 };
+
+/**
+ * POST: 204 = undone; 404 MERGE_NOT_FOUND, 409 UNDO_NOT_AVAILABLE / NEWER_MERGE, 410 UNDO_EXPIRED (body `{code}`),
+ * 403 below AUDITOR.
+ */
+export const undoMerge = (mergeId) => apiFetch(`${base()}/merges/${mergeId}/undo`, {method: 'POST'});