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.

components/settings/LibraryOptionsSheet.tsx

@@ -229,7 +229,7 @@ export const LibraryOptionsSheet: React.FC<Props> = ({
             />
           </OptionGroup>
 
-          <OptionGroup title='Options'>
+          <OptionGroup title={t("library.options.options_title")}>
             <ToggleItem
               label={t("library.options.show_titles")}
               value={settings.showTitles}