feat: manage, account and help pages

- 管理页：单元集 / 计划 / 算法三个维度，卡片带元数据
- 账户页：资料、学习数据、竖直分组的四个功能入口
- 新增设置、文档与帮助（底部可上滑助手）、反馈、关于四个页面
- 抽出 StatBlock 与 GroupedListSection 共用组件
- 关闭 Android 预测式返回，转场改为 M3 水平滑入

lib/models/account_profile.dart

@@ -0,0 +1,30 @@
+/// 账户页展示的用户资料。
+class AccountProfile {
+  const AccountProfile({
+    required this.displayName,
+    required this.userId,
+    required this.stage,
+    required this.totalHours,
+    required this.streakDays,
+  });
+
+  final String displayName;
+  final String userId;
+
+  /// 学习阶段，如「大一」。
+  final String stage;
+
+  /// 累计学习时长（小时）。
+  final int totalHours;
+
+  /// 连续学习天数。
+  final int streakDays;
+
+  static const AccountProfile sample = AccountProfile(
+    displayName: 'zhiyu',
+    userId: 'KM-1024',
+    stage: '大一',
+    totalHours: 42,
+    streakDays: 4,
+  );
+}

lib/models/catalog.dart

@@ -0,0 +1,146 @@
+/// 管理页「单元集」列表项。
+class DeckInfo {
+  const DeckInfo({
+    required this.id,
+    required this.title,
+    required this.algorithm,
+    required this.totalItems,
+    required this.activatedItems,
+    required this.lastStudied,
+    required this.description,
+  });
+
+  final String id;
+  final String title;
+  final String algorithm;
+  final int totalItems;
+  final int activatedItems;
+
+  /// 最近一次学习的人话描述，如「3 天前」。
+  final String lastStudied;
+  final String description;
+
+  double get progress => totalItems == 0 ? 0 : activatedItems / totalItems;
+
+  String get progressPercent => '${(progress * 100).toStringAsFixed(1)}%';
+
+  static const List<DeckInfo> samples = <DeckInfo>[
+    DeckInfo(
+      id: 'cngk-t',
+      title: '高考必背古诗文',
+      algorithm: 'NSP-0',
+      totalItems: 950,
+      activatedItems: 295,
+      lastStudied: '3 天前',
+      description: '高考古诗文 60 篇',
+    ),
+    DeckInfo(
+      id: 'cet6-core',
+      title: 'CET-6 核心词汇',
+      algorithm: 'SM-2',
+      totalItems: 6204,
+      activatedItems: 2819,
+      lastStudied: '昨天',
+      description: '六级高频词，按词频排序',
+    ),
+    DeckInfo(
+      id: 'linalg-formula',
+      title: '线性代数公式',
+      algorithm: 'FSRS',
+      totalItems: 120,
+      activatedItems: 0,
+      lastStudied: '未开始',
+      description: '期末复习用，含推导步骤',
+    ),
+  ];
+}
+
+/// 管理页「计划」列表项。
+class PlanInfo {
+  const PlanInfo({
+    required this.title,
+    required this.newPerDay,
+    required this.reviewPerDay,
+    required this.window,
+    required this.enabled,
+    required this.description,
+  });
+
+  final String title;
+  final int newPerDay;
+  final int reviewPerDay;
+
+  /// 允许学习的时段，如「08:00–23:00」。
+  final String window;
+  final bool enabled;
+  final String description;
+
+  static const List<PlanInfo> samples = <PlanInfo>[
+    PlanInfo(
+      title: '每日计划',
+      newPerDay: 35,
+      reviewPerDay: 200,
+      window: '08:00–23:00',
+      enabled: true,
+      description: '默认计划，每天引入 35 条新识记',
+    ),
+    PlanInfo(
+      title: '周末冲刺',
+      newPerDay: 0,
+      reviewPerDay: 400,
+      window: '09:00–22:00',
+      enabled: false,
+      description: '只复习不新增，周末启用',
+    ),
+  ];
+}
+
+/// 管理页「算法」列表项。
+class AlgorithmInfo {
+  const AlgorithmInfo({
+    required this.name,
+    required this.version,
+    required this.description,
+    required this.parameters,
+    required this.isDefault,
+  });
+
+  final String name;
+  final String version;
+  final String description;
+
+  /// 关键参数摘要，如「阈值 3 · 间隔上限 365 天」。
+  final String parameters;
+  final bool isDefault;
+
+  static const List<AlgorithmInfo> samples = <AlgorithmInfo>[
+    AlgorithmInfo(
+      name: 'NSP-0',
+      version: '1.0',
+      description: '快速筛选用非间隔重复调度器',
+      parameters: '阈值 3 · 每日复习',
+      isDefault: true,
+    ),
+    AlgorithmInfo(
+      name: 'SM-2',
+      version: '1.0',
+      description: '第二代 SuperMemo，Anki 的默认调度器',
+      parameters: 'EF 1.3–2.5 · 间隔倍数 1.0',
+      isDefault: false,
+    ),
+    AlgorithmInfo(
+      name: 'FSRS',
+      version: '5.0',
+      description: '先进开放间隔重复调度器',
+      parameters: '可保留率 0.9 · 权重 19',
+      isDefault: false,
+    ),
+    AlgorithmInfo(
+      name: 'SM-15M',
+      version: '1.0',
+      description: '复杂间隔重复调度器（逆向工程实现）',
+      parameters: '最优间隔 · 遗忘曲线拟合',
+      isDefault: false,
+    ),
+  ];
+}