📱 refactor(web): remove legacy isMobile util and migrate to useIsMobile hook

BREAKING CHANGE:
helpers/utils.js no longer exports `isMobile()`.
Any external code that relied on this function must switch to the `useIsMobile` React hook.

Summary
-------
1. Deleted the obsolete `isMobile()` function from helpers/utils.js.
2. Introduced `MOBILE_BREAKPOINT` constant and `matchMedia`-based detection for non-React contexts.
3. Reworked toast positioning logic in utils.js to rely on `matchMedia`.
4. Updated render.js:
   • Removed isMobile import.
   • Added MOBILE_BREAKPOINT detection in `truncateText`.
5. Migrated every page/component to the `useIsMobile` hook:
   • Layout: HeaderBar, PageLayout, SiderBar
   • Pages: Home, Detail, Playground, User (Add/Edit), Token, Channel, Redemption, Ratio Sync
   • Components: ChannelsTable, ChannelSelectorModal, ConflictConfirmModal
6. Purged all remaining `isMobile()` calls and legacy imports.
7. Added missing `const isMobile = useIsMobile()` declarations where required.

Benefits
--------
• Unifies mobile detection with a React-friendly hook.
• Eliminates duplicated logic and improves maintainability.
• Keeps non-React helpers lightweight by using `matchMedia` directly.

web/src/pages/Setting/Ratio/UpstreamRatioSync.js

@@ -18,7 +18,8 @@ import {
   AlertTriangle,
   CheckCircle,
 } from 'lucide-react';
-import { API, showError, showSuccess, showWarning, stringToColor, isMobile } from '../../../helpers';
+import { API, showError, showSuccess, showWarning, stringToColor } from '../../../helpers';
+import { useIsMobile } from '../../../hooks/useIsMobile.js';
 import { DEFAULT_ENDPOINT } from '../../../constants';
 import { useTranslation } from 'react-i18next';
 import {
@@ -28,6 +29,7 @@ import {
 import ChannelSelectorModal from '../../../components/settings/ChannelSelectorModal';
 
 function ConflictConfirmModal({ t, visible, items, onOk, onCancel }) {
+  const isMobile = useIsMobile();
   const columns = [
     { title: t('渠道'), dataIndex: 'channel' },
     { title: t('模型'), dataIndex: 'model' },
@@ -49,7 +51,7 @@ function ConflictConfirmModal({ t, visible, items, onOk, onCancel }) {
       visible={visible}
       onCancel={onCancel}
       onOk={onOk}
-      size={isMobile() ? 'full-width' : 'large'}
+      size={isMobile ? 'full-width' : 'large'}
     >
       <Table columns={columns} dataSource={items} pagination={false} size="small" />
     </Modal>
@@ -61,6 +63,7 @@ export default function UpstreamRatioSync(props) {
   const [modalVisible, setModalVisible] = useState(false);
   const [loading, setLoading] = useState(false);
   const [syncLoading, setSyncLoading] = useState(false);
+  const isMobile = useIsMobile();
 
   // 渠道选择相关
   const [allChannels, setAllChannels] = useState([]);