feat(app): 初始化 Flutter 前端项目 (Phase F0)

- Flutter 3.44.0 + Dart 3.12.0
- 设计系统: 7色Token×浅/深模式, NotoSansSC/Caveat字体, 圆角10/16/22/28/pill, 三级阴影
- ResponsiveScaffold: 手机底部TabBar / 平板侧边Rail / 桌面三栏
- go_router 路由表: 13个页面 (5个Tab + 8个全屏页面)
- 13个功能模块占位页面 (home/calendar/mood/search/profile/editor/auth/class/teacher/parent/achievement/stickers/templates)
- 依赖: flutter_bloc, go_router, freezed, isar, dio, perfect_freehand, fl_chart
- 中国镜像: PUB_HOSTED_URL + FLUTTER_STORAGE_BASE_URL
- flutter analyze: No issues found

app/lib/features/class_/views/class_page.dart

@@ -0,0 +1,14 @@
+import 'package:flutter/material.dart';
+
+class ClassPage extends StatelessWidget {
+  const ClassPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: Center(
+        child: Text('班级 - 占位页面'),
+      ),
+    );
+  }
+}