refactor(stores): unify data sources pattern and extract BaseManager base class

- Add BaseManager/CachedManager base classes to eliminate duplicate cache logic
- Add postListSources.ts with IPostListPagedSource interface
- Add groupListSources.ts with IGroupListPagedSource interface
- Refactor postManager to use CachedManager and Sources pattern
- Refactor groupManager to use CachedManager and Sources pattern
- Clean up duplicate methods in groupService.ts
- Fix TypeScript errors and remove mock data

src/screens/material/MaterialsScreen.tsx

@@ -122,7 +122,7 @@ export const MaterialsScreen: React.FC = () => {
     if (isLoading) {
       return (
         <View style={styles.centerContainer}>
-          <Loading size="large" />
+          <Loading size="lg" />
         </View>
       );
     }
@@ -165,7 +165,7 @@ export const MaterialsScreen: React.FC = () => {
           </TouchableOpacity>
         </View>
         <View style={styles.headerCenter}>
-          <Text style={[styles.headerTitle, isWideScreen && styles.headerTitleWide]}>
+          <Text style={isWideScreen ? [styles.headerTitle, styles.headerTitleWide] : styles.headerTitle}>
             学习资料
           </Text>
         </View>