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/test/widget_test.dart

@@ -0,0 +1,10 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:nuanji_app/app.dart';
+
+void main() {
+  testWidgets('App smoke test', (WidgetTester tester) async {
+    await tester.pumpWidget(const NuanjiApp());
+    // 验证 app 启动无崩溃
+    expect(find.text('首页'), findsWidgets);
+  });
+}