新增阅读器功能，待优化

lib/utils/book_file_helper.dart

@@ -0,0 +1,71 @@
+import 'dart:io';
+import 'package:path_provider/path_provider.dart';
+import 'package:path/path.dart' as p;
+
+/// 阅读器文件路径管理
+class BookFileHelper {
+  static final BookFileHelper instance = BookFileHelper._init();
+  BookFileHelper._init();
+
+  String? _rootPath;
+
+  Future<String> get _root async {
+    if (_rootPath != null) return _rootPath!;
+    final dir = await getApplicationDocumentsDirectory();
+    _rootPath = p.join(dir.path, 'mooknote', 'book_file');
+    await Directory(_rootPath!).create(recursive: true);
+    return _rootPath!;
+  }
+
+  Future<String> get bookFileRoot async => _root;
+
+  Future<String> get coverDir async {
+    final root = await _root;
+    final dir = p.join(root, 'cover');
+    await Directory(dir).create(recursive: true);
+    return dir;
+  }
+
+  Future<String> bookDir(String bookId) async {
+    final root = await _root;
+    final dir = p.join(root, bookId);
+    await Directory(dir).create(recursive: true);
+    return dir;
+  }
+
+  Future<String> bookFile(String bookId, String fileName) async {
+    final dir = await bookDir(bookId);
+    return p.join(dir, fileName);
+  }
+
+  String? relativePath(String absolutePath) {
+    if (_rootPath == null) return null;
+    if (absolutePath.startsWith(_rootPath!)) {
+      return absolutePath.substring(_rootPath!.length + 1);
+    }
+    return null;
+  }
+
+  Future<String> absolutePath(String relativePath) async {
+    final root = await _root;
+    return p.join(root, relativePath);
+  }
+
+  Future<void> deleteBookFiles(String bookId) async {
+    final dir = await bookDir(bookId);
+    if (await Directory(dir).exists()) {
+      await Directory(dir).delete(recursive: true);
+    }
+  }
+
+  /// 同步初始化（必须在使用 resolveAbsolutePath 前调用一次 bookFileRoot）
+  Future<void> ensureInitialized() async {
+    await _root;
+  }
+
+  /// 根据相对路径解析绝对路径（调用前需确保已初始化）
+  String resolveAbsolutePath(String relativePath) {
+    if (_rootPath == null) return relativePath;
+    return p.join(_rootPath!, relativePath);
+  }
+}