Slide thumbnail shows hardcoded Chinese "加载中 ..." in every locale

`components/slide-renderer/SlideThumbnail.tsx` renders a hardcoded `加载中 ...` placeholder while a thumbnail is not yet visible (`if (!visible)` branch, line ~240). Every non-Chinese locale shows Chinese text in the workbench page strip during loading. The component already has `const { t } = useI18n()` and every locale file has `common.loading`.