feat: 落地登录纵切——网络层、认证会话与登录/注册/Splash 三页（ADR-003/ADR-004）

- 新增 lib/core/network/：ApiClient（dio 封装，错误信封→类型化异常，base URL 经 --dart-define=PATBOND_API_BASE_URL 注入）、AuthInterceptor（Bearer + 设备标识）、TokenRefresher（单飞刷新，401/40101 重放一次，仅 40102/再 401 清会话）
- 新增 lib/features/auth/：SessionManager（token 只进 flutter_secure_storage）、AuthRepository（login/register/refresh/logout/me，注册带 Idempotency-Key）、Splash（500ms 最短停留/5s 超时/错误态重试+改用账号登录）、登录页与注册页（照 12 号组装稿，错误三层映射，预留区不渲染）
- App 根组件改为认证状态机驱动（Splash↔登录↔主壳 300ms fade）；个人中心退出登录接入真实 logout
- 顺带修复：FIX-1 促销卡渐变改 [primaryStrong, primary]、FIX-2 补 helperStyle: muted、README dart format 命令补 --output=none
- 新增依赖：dio ^5.11.1、flutter_secure_storage ^11.0.0、uuid ^4.6.0
- 门禁：dart format（0 changed）/ flutter analyze（No issues）/ flutter test（30 passed，其中新增 23：TokenRefresher 5 + AuthRepository 10 + 登录页 4 + 注册页 4）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/features/auth/auth_models.dart

@@ -0,0 +1,57 @@
+/// 认证接口的响应模型（接口契约冻结稿，字段名与后端一致）。
+class AuthTokens {
+  const AuthTokens({
+    required this.userId,
+    required this.tokenType,
+    required this.accessToken,
+    required this.accessTokenExpiresAt,
+    required this.refreshToken,
+    required this.refreshTokenExpiresAt,
+  });
+
+  factory AuthTokens.fromJson(Map<String, dynamic> json) {
+    return AuthTokens(
+      userId: json['userId'] as String,
+      tokenType: json['tokenType'] as String,
+      accessToken: json['accessToken'] as String,
+      accessTokenExpiresAt: DateTime.parse(
+        json['accessTokenExpiresAt'] as String,
+      ),
+      refreshToken: json['refreshToken'] as String,
+      refreshTokenExpiresAt: DateTime.parse(
+        json['refreshTokenExpiresAt'] as String,
+      ),
+    );
+  }
+
+  final String userId;
+  final String tokenType;
+  final String accessToken;
+  final DateTime accessTokenExpiresAt;
+  final String refreshToken;
+  final DateTime refreshTokenExpiresAt;
+}
+
+/// `GET /api/v1/me` 的用户资料。
+class UserProfile {
+  const UserProfile({
+    required this.userId,
+    required this.username,
+    required this.phone,
+    required this.createdAt,
+  });
+
+  factory UserProfile.fromJson(Map<String, dynamic> json) {
+    return UserProfile(
+      userId: json['userId'] as String,
+      username: json['username'] as String,
+      phone: json['phone'] as String,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  final String userId;
+  final String username;
+  final String phone;
+  final DateTime createdAt;
+}