i18n: make two hardcoded titles translatable

Replace hardcoded English strings with t() and add the source keys:
- LibraryOptionsSheet 'Options' -> library.options.options_title
- MpvPlayerView.web iframe 'MPV Player' -> player.mpv_player_title
New keys only (English source); other locales fall back to English until translated via Crowdin, so no regression.

modules/mpv-player/src/MpvPlayerView.web.tsx

@@ -1,11 +1,13 @@
+import { useTranslation } from "react-i18next";
 import { MpvPlayerViewProps } from "./MpvPlayer.types";
 
 export default function MpvPlayerView(props: MpvPlayerViewProps) {
   const url = props.source?.url ?? "";
+  const { t } = useTranslation();
   return (
     <div>
       <iframe
-        title='MPV Player'
+        title={t("player.mpv_player_title")}
         style={{ flex: 1 }}
         src={url}
         onLoad={() => props.onLoad?.({ nativeEvent: { url } })}