📱 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/components/settings/ChannelSelectorModal.js

@@ -1,5 +1,5 @@
 import React, { useState, useEffect, forwardRef, useImperativeHandle } from 'react';
-import { isMobile } from '../../helpers';
+import { useIsMobile } from '../../hooks/useIsMobile.js';
 import {
   Modal,
   Table,
@@ -26,6 +26,7 @@ const ChannelSelectorModal = forwardRef(({
   const [searchText, setSearchText] = useState('');
   const [currentPage, setCurrentPage] = useState(1);
   const [pageSize, setPageSize] = useState(10);
+  const isMobile = useIsMobile();
 
   const [filteredData, setFilteredData] = useState([]);
 
@@ -186,7 +187,7 @@ const ChannelSelectorModal = forwardRef(({
       onCancel={onCancel}
       onOk={onOk}
       title={<span className="text-lg font-semibold">{t('选择同步渠道')}</span>}
-      size={isMobile() ? 'full-width' : 'large'}
+      size={isMobile ? 'full-width' : 'large'}
       keepDOM
       lazyRender={false}
     >