Introduce a runtime UI language switcher and add Japanese/Korean locale support. Added _locales/ja and _locales/ko messages, updated en and zh_CN message files with new language-related keys and sendImage label. Implemented dynamic i18n handling: new i18n.js supports override messages, persistent uiLanguage in storage, and emits i18n:changed; background.js now loads override locales, updates context menu titles and listens for storage changes. Integrated dayjs locales (js/ja.js, js/ko.js) and made oper.js use a unified msg() helper and react to language changes. Added language selector UI in popup.html and styling in css/main.css.
jonny