Add «Скачать .docx» (Word) and «Скачать .pptx» (PowerPoint) buttons to
ForecastExportButtons, alongside the existing .md/.json/Telegram controls.
The backend /forecast/export already renders both formats; they were just
not reachable from the UI. Both reuse the existing response.blob() →
triggerDownload path (correct for binary zip-based OOXML — never .text(),
which would corrupt the archive). Per-format loading state; 404 →
«Отчёт ещё не готов». No new deps (icons from lucide-react).
Part of #958.