Add UI language switcher and ja/ko locales

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.

_locales/zh_CN/messages.json

@@ -145,5 +145,23 @@
    },
    "tipSend": {
       "message": "发送（Ctrl/⌘+Enter）"
+   },
+   "tipLanguage": {
+      "message": "语言"
+   },
+   "langAuto": {
+      "message": "跟随浏览器"
+   },
+   "langEnglish": {
+      "message": "English"
+   },
+   "langChineseSimplified": {
+      "message": "简体中文"
+   },
+   "langJapanese": {
+      "message": "日本語"
+   },
+   "langKorean": {
+      "message": "한국어"
    }
 }