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/widgets/grouped_list_section.dart

@@ -0,0 +1,117 @@
+import 'package:flutter/material.dart';
+
+/// 一组圆角卡片式列表项，行间自动加分隔线。
+///
+/// 容器用 [Material] 而不是 `Container`：`InkWell` 的水波纹画在最近的
+/// `Material` 祖先上，普通容器会让墨水落到外层 Scaffold，圆角裁剪失效，
+/// 按住时就变成直角矩形。
+class GroupedListSection extends StatelessWidget {
+  const GroupedListSection({
+    required this.tiles,
+    this.dividerIndent = 58,
+    super.key,
+  });
+
+  final List<Widget> tiles;
+
+  /// 分隔线左侧缩进，默认与图标后的文字对齐。
+  final double dividerIndent;
+
+  @override
+  Widget build(BuildContext context) {
+    final ColorScheme colors = Theme.of(context).colorScheme;
+
+    return Material(
+      color: colors.surfaceContainerLow,
+      borderRadius: BorderRadius.circular(20),
+      clipBehavior: Clip.antiAlias,
+      child: Column(
+        children: <Widget>[
+          for (final (int index, Widget tile) in tiles.indexed) ...<Widget>[
+            tile,
+            if (index != tiles.length - 1)
+              Divider(
+                height: 1,
+                thickness: 1,
+                indent: dividerIndent,
+                color: colors.outlineVariant.withValues(alpha: 0.6),
+              ),
+          ],
+        ],
+      ),
+    );
+  }
+}
+
+/// 分组列表里的一行：图标 + 标题（+ 可选副标题）+ 右侧箭头。
+class GroupedListTile extends StatelessWidget {
+  const GroupedListTile({
+    required this.icon,
+    required this.title,
+    this.subtitle,
+    this.onTap,
+    this.trailing,
+    super.key,
+  });
+
+  final IconData icon;
+  final String title;
+  final String? subtitle;
+  final VoidCallback? onTap;
+
+  /// 右侧自定义内容；不传且有 [onTap] 时显示箭头。
+  final Widget? trailing;
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    final ColorScheme colors = theme.colorScheme;
+    final String? subtitle = this.subtitle;
+    final Widget? trailing = this.trailing;
+
+    return InkWell(
+      onTap: onTap,
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 18, vertical: 18),
+        child: Row(
+          children: <Widget>[
+            Icon(icon, size: 22, color: colors.primary),
+            const SizedBox(width: 18),
+            Expanded(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: <Widget>[
+                  Text(
+                    title,
+                    style: theme.textTheme.bodyLarge,
+                    maxLines: 1,
+                    overflow: TextOverflow.ellipsis,
+                  ),
+                  if (subtitle != null) ...<Widget>[
+                    const SizedBox(height: 2),
+                    Text(
+                      subtitle,
+                      style: theme.textTheme.bodySmall?.copyWith(
+                        color: colors.onSurfaceVariant,
+                      ),
+                      maxLines: 1,
+                      overflow: TextOverflow.ellipsis,
+                    ),
+                  ],
+                ],
+              ),
+            ),
+            if (trailing != null)
+              trailing
+            else if (onTap != null)
+              Icon(
+                Icons.chevron_right_rounded,
+                size: 22,
+                color: colors.onSurfaceVariant,
+              ),
+          ],
+        ),
+      ),
+    );
+  }
+}

lib/widgets/stat_block.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+
+/// 「标签 + 大数字 +（可选）单位」的组合，学习页与账户页共用。
+class StatBlock extends StatelessWidget {
+  const StatBlock({
+    required this.label,
+    required this.value,
+    this.unit,
+    super.key,
+  });
+
+  final String label;
+  final String value;
+  final String? unit;
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    final ColorScheme colors = theme.colorScheme;
+    final String? unit = this.unit;
+
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: <Widget>[
+        Text(
+          label,
+          style: theme.textTheme.bodyMedium?.copyWith(
+            color: colors.onSurfaceVariant,
+          ),
+        ),
+        const SizedBox(height: 2),
+        Row(
+          crossAxisAlignment: CrossAxisAlignment.baseline,
+          textBaseline: TextBaseline.alphabetic,
+          children: <Widget>[
+            Flexible(
+              child: Text(
+                value,
+                maxLines: 1,
+                overflow: TextOverflow.ellipsis,
+                style: theme.textTheme.displaySmall?.copyWith(
+                  fontWeight: FontWeight.w700,
+                  height: 1.1,
+                ),
+              ),
+            ),
+            if (unit != null) ...<Widget>[
+              const SizedBox(width: 6),
+              Text(
+                unit,
+                style: theme.textTheme.bodySmall?.copyWith(
+                  color: colors.onSurfaceVariant,
+                ),
+              ),
+            ],
+          ],
+        ),
+      ],
+    );
+  }
+}