feat(缓存): 添加本地缓存功能并优化网络状态提示

实现本地缓存管理器，支持预加载开关和排行榜数据缓存
优化网络状态提示文案和错误处理
移除调试日志打印，改进用户体验

lib/views/home/home_page.dart

@@ -191,7 +191,6 @@ class _HomePageState extends State<HomePage>
         }
       }
     } catch (e) {
-      debugPrint('加载诗词失败: $e');
       if (mounted) {
         setState(() {
           _loading = false;
@@ -269,7 +268,6 @@ class _HomePageState extends State<HomePage>
         }
       }
     } catch (e) {
-      debugPrint('加载指定诗词失败: $e');
       if (mounted) {
         setState(() {
           _poetryData = _createDefaultPoetryData();
@@ -500,7 +498,7 @@ class _HomePageState extends State<HomePage>
       if (mounted) {
         PoetryStateManager.showSnackBar(
           context,
-          '加载下一条失败',
+          '加载下一条失败，建议开启离线模式',
           backgroundColor: AppConstants.errorColor,
           duration: const Duration(milliseconds: 200),
         );