♻️ refactor(web): migrate React modules from .js to .jsx and align entrypoint

- Rename React components/pages/utilities that contain JSX to `.jsx` across `web/src`
- Update import paths and re-exports to match new `.jsx` extensions
- Fix Vite entry by switching `web/index.html` from `/src/index.js` to `/src/index.jsx`
- Verified remaining `.js` files are plain JS (hooks/helpers/constants) and do not require JSX
- No runtime behavior changes; extension and reference alignment only

Context: Resolves the Vite pre-transform error caused by the stale `/src/index.js` entry after migrating to `.jsx`.

web/src/components/layout/HeaderBar/LanguageSelector.jsx

@@ -0,0 +1,59 @@
+/*
+Copyright (C) 2025 QuantumNous
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, either version 3 of the
+License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+For commercial licensing, please contact support@quantumnous.com
+*/
+
+import React from 'react';
+import { Button, Dropdown } from '@douyinfe/semi-ui';
+import { IconLanguage } from '@douyinfe/semi-icons';
+import { CN, GB } from 'country-flag-icons/react/3x2';
+
+const LanguageSelector = ({ currentLang, onLanguageChange, t }) => {
+  return (
+    <Dropdown
+      position="bottomRight"
+      render={
+        <Dropdown.Menu className="!bg-semi-color-bg-overlay !border-semi-color-border !shadow-lg !rounded-lg dark:!bg-gray-700 dark:!border-gray-600">
+          <Dropdown.Item
+            onClick={() => onLanguageChange('zh')}
+            className={`!flex !items-center !gap-2 !px-3 !py-1.5 !text-sm !text-semi-color-text-0 dark:!text-gray-200 ${currentLang === 'zh' ? '!bg-semi-color-primary-light-default dark:!bg-blue-600 !font-semibold' : 'hover:!bg-semi-color-fill-1 dark:hover:!bg-gray-600'}`}
+          >
+            <CN title="中文" className="!w-5 !h-auto" />
+            <span>中文</span>
+          </Dropdown.Item>
+          <Dropdown.Item
+            onClick={() => onLanguageChange('en')}
+            className={`!flex !items-center !gap-2 !px-3 !py-1.5 !text-sm !text-semi-color-text-0 dark:!text-gray-200 ${currentLang === 'en' ? '!bg-semi-color-primary-light-default dark:!bg-blue-600 !font-semibold' : 'hover:!bg-semi-color-fill-1 dark:hover:!bg-gray-600'}`}
+          >
+            <GB title="English" className="!w-5 !h-auto" />
+            <span>English</span>
+          </Dropdown.Item>
+        </Dropdown.Menu>
+      }
+    >
+      <Button
+        icon={<IconLanguage className="text-lg" />}
+        aria-label={t('切换语言')}
+        theme="borderless"
+        type="tertiary"
+        className="!p-1.5 !text-current focus:!bg-semi-color-fill-1 dark:focus:!bg-gray-700 !rounded-full !bg-semi-color-fill-0 dark:!bg-semi-color-fill-1 hover:!bg-semi-color-fill-1 dark:hover:!bg-semi-color-fill-2"
+      />
+    </Dropdown>
+  );
+};
+
+export default LanguageSelector;