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/app.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+import 'core/theme/app_theme.dart';
+import 'core/routing/app_router.dart';
+
+class NuanjiApp extends StatelessWidget {
+  const NuanjiApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp.router(
+      title: '暖记',
+      debugShowCheckedModeBanner: false,
+      theme: AppTheme.light(),
+      darkTheme: AppTheme.dark(),
+      themeMode: ThemeMode.system,
+      routerConfig: appRouter,
+    );
+  }
+}