Add channels management to Sidebar and API index; update MaterialSubjects for icon handling

src/pages/Materials.tsx

@@ -117,7 +117,10 @@ export default function Materials() {
   const loadSubjects = useCallback(async () => {
     try {
       const response = await subjectsApi.getSubjects()
-      setSubjects(response.data.data || [])
+      const data = response.data.data
+      // 后端返回 { list: [...] } 结构
+      const list = Array.isArray(data) ? data : (data?.list || [])
+      setSubjects(list)
     } catch (error) {
       console.error('Failed to load subjects:', error)
     }