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/core/theme/app_shadows.dart

@@ -0,0 +1,49 @@
+// 暖记阴影系统 — soft / medium / float
+
+import 'package:flutter/material.dart';
+
+class AppShadows {
+  AppShadows._();
+
+  /// 柔和阴影 — 卡片默认
+  static List<BoxShadow> soft(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    return [
+      BoxShadow(
+        color: isDark
+            ? Colors.black.withValues(alpha: 0.3)
+            : const Color(0xFF2D2420).withValues(alpha: 0.08),
+        offset: const Offset(0, 2),
+        blurRadius: 8,
+      ),
+    ];
+  }
+
+  /// 中等阴影 — 浮动元素、FAB
+  static List<BoxShadow> medium(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    return [
+      BoxShadow(
+        color: isDark
+            ? Colors.black.withValues(alpha: 0.4)
+            : const Color(0xFF2D2420).withValues(alpha: 0.12),
+        offset: const Offset(0, 4),
+        blurRadius: 16,
+      ),
+    ];
+  }
+
+  /// 浮动阴影 — 弹窗、底部面板
+  static List<BoxShadow> floating(BuildContext context) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    return [
+      BoxShadow(
+        color: isDark
+            ? Colors.black.withValues(alpha: 0.5)
+            : const Color(0xFF2D2420).withValues(alpha: 0.16),
+        offset: const Offset(0, 8),
+        blurRadius: 24,
+      ),
+    ];
+  }
+}