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/constants/breakpoints.dart

@@ -0,0 +1,24 @@
+// 暖记响应式断点
+// 手机 < 600 | 平板 600-1024 | 桌面 > 1024
+
+class Breakpoints {
+  Breakpoints._();
+
+  /// 手机最大宽度
+  static const double mobile = 600;
+
+  /// 平板最大宽度
+  static const double tablet = 1024;
+
+  /// 触摸目标最小尺寸 (WCAG + Apple HIG)
+  static const double touchTarget = 44;
+
+  /// 判断设备类型
+  static DeviceType getDeviceType(double width) {
+    if (width < mobile) return DeviceType.mobile;
+    if (width < tablet) return DeviceType.tablet;
+    return DeviceType.desktop;
+  }
+}
+
+enum DeviceType { mobile, tablet, desktop }

app/lib/core/constants/design_tokens.dart

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+// 暖记设计常量 — 间距 / 动画 / 触摸目标
+
+import 'package:flutter/animation.dart';
+
+class DesignTokens {
+  DesignTokens._();
+
+  // ===== 间距 =====
+  static const double spacing4 = 4;
+  static const double spacing8 = 8;
+  static const double spacing12 = 12;
+  static const double spacing16 = 16;
+  static const double spacing20 = 20;
+  static const double spacing24 = 24;
+  static const double spacing32 = 32;
+  static const double spacing48 = 48;
+
+  // ===== 动画时长 =====
+  static const Duration animFast = Duration(milliseconds: 150);
+  static const Duration animNormal = Duration(milliseconds: 300);
+  static const Duration animSlow = Duration(milliseconds: 500);
+
+  // ===== 弹性曲线 cubic-bezier(0.34, 1.56, 0.64, 1) =====
+  static const Curve warmCurve = Curves.easeOutBack;
+
+  // ===== 列表/网格 =====
+  static const int journalGridCrossAxisCountMobile = 2;
+  static const int journalGridCrossAxisCountTablet = 3;
+  static const int journalGridCrossAxisCountDesktop = 4;
+
+  // ===== 日记限制 =====
+  static const int maxTagsPerJournal = 10;
+  static const int maxTitleLength = 100;
+  static const int maxStrokesPerElement = 5000;
+  static const int maxUndoSteps = 50;
+
+  // ===== 班级码 =====
+  static const int classCodeLength = 6;
+  static const int classCodeMaxAttempts = 5;
+  static const int classCodeLockoutMinutes = 30;
+}