feat: custom pwd of bak (#827)

lib/data/model/app/bak/backup_source.dart

@@ -0,0 +1,62 @@
+import 'dart:io';
+
+import 'package:fl_lib/fl_lib.dart';
+import 'package:flutter/material.dart';
+
+/// Abstract interface for backup content sources
+abstract class BackupSource {
+  /// Get content from this source for restore
+  Future<String?> getContent();
+  
+  /// Save content to this source for backup
+  Future<void> saveContent(String filePath);
+  
+  /// Display name for this source
+  String get displayName;
+  
+  /// Icon for this source
+  IconData get icon;
+}
+
+/// File-based backup source
+class FileBackupSource implements BackupSource {
+  @override
+  Future<String?> getContent() async {
+    return await Pfs.pickFileString();
+  }
+
+  @override
+  Future<void> saveContent(String filePath) async {
+    await Pfs.sharePaths(paths: [filePath]);
+  }
+
+  @override
+  String get displayName => libL10n.file;
+
+  @override
+  IconData get icon => Icons.file_open;
+}
+
+/// Clipboard-based backup source
+class ClipboardBackupSource implements BackupSource {
+  @override
+  Future<String?> getContent() async {
+    final text = await Pfs.paste();
+    if (text == null || text.isEmpty) {
+      return null;
+    }
+    return text.trim();
+  }
+
+  @override
+  Future<void> saveContent(String filePath) async {
+    final content = await File(filePath).readAsString();
+    Pfs.copy(content);
+  }
+
+  @override
+  String get displayName => libL10n.clipboard;
+
+  @override
+  IconData get icon => Icons.content_paste;
+}