feat: add local filesystem sync service

[landeqiming666]

Summary

• add a local filesystem sync service that can export the existing sync file payload to a selected folder
• add import support for reading sync files back from a selected folder
• add a local sync settings model for the folder path stored in syncLocalSettings.json
• cover the new service with focused unit tests

Context

Related to #256.

This is a small foundation for the file-system sync mode requested in the issue. It avoids changing the existing Gist upload/download flow until the storage backend is test-covered.

Validation

• npm run tslint-check
• ./node_modules/.bin/mocha --recursive "./out/test/service/fileService/.js" "./out/test/service/localSyncService/.js"
• git diff --check

Note: full tsc currently hits the repository's existing fetch / @octokit type baseline after dependency install; the new service and adjacent file service tests pass.

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #256: Adding Support for Import / Export Sync Settings to File System

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