feat(apps): establish shared mobile design system

apps/heqi_design_system/lib/src/tokens.dart

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+import 'package:flutter/material.dart';
+
+/// 两端品牌主题。安全语义色始终共享，不能被品牌色覆盖。
+enum HeqiBrand { consumer, service }
+
+abstract final class HeqiColors {
+  static const consumerPrimary = Color(0xFF2563EB);
+  static const servicePrimary = Color(0xFF4F46E5);
+  static const success = Color(0xFF16875D);
+  static const warning = Color(0xFFB86400);
+  static const danger = Color(0xFFC7352A);
+
+  static const lightBackground = Color(0xFFF7F8FA);
+  static const darkBackground = Color(0xFF0B1220);
+  static const darkSurface = Color(0xFF111827);
+}
+
+/// 4dp 基础网格。页面不得新增不属于本表的随意间距。
+abstract final class HeqiSpacing {
+  static const x1 = 4.0;
+  static const x2 = 8.0;
+  static const x3 = 12.0;
+  static const x4 = 16.0;
+  static const x5 = 20.0;
+  static const x6 = 24.0;
+  static const x7 = 28.0;
+  static const x8 = 32.0;
+  static const x10 = 40.0;
+  static const x12 = 48.0;
+}
+
+abstract final class HeqiRadius {
+  static const small = 10.0;
+  static const medium = 12.0;
+  static const large = 16.0;
+  static const extraLarge = 20.0;
+  static const sheet = 24.0;
+}
+
+abstract final class HeqiSize {
+  static const minTouchTarget = 48.0;
+  static const buttonHeight = 52.0;
+  static const navigationBarHeight = 72.0;
+  static const contentMaxWidth = 720.0;
+  static const iconSmall = 16.0;
+  static const iconMedium = 24.0;
+  static const iconLarge = 32.0;
+}
+
+abstract final class HeqiMotion {
+  static const fast = Duration(milliseconds: 150);
+  static const standard = Duration(milliseconds: 220);
+  static const emphasized = Duration(milliseconds: 300);
+}
+
+abstract final class HeqiBreakpoints {
+  static const compact = 600.0;
+  static const medium = 840.0;
+  static const expanded = 1024.0;
+}