[Bug]: Data loss during sync

### Prerequisites|前提条件

- [x] I have checked the [troubleshooting](https://github.com/Anxcye/anx-reader/blob/develop/docs/troubleshooting.md)|我已经查看过[故障排除](https://github.com/Anxcye/anx-reader/blob/develop/docs/troubleshooting.md)
- [x] I have searched for similar issues and did not find any|我已经搜索过没有相同的问题

### Describe the bug|描述问题

I 've had severall loss of data during sync with webdav. It took me some time to understand that there is no sync at all, it is an upload/download function of all the databse...

Typical usecase where I loose data : I read and annotate a book on a portable device in the subway/metro without internet connection. When I get to work, I greet my colleagues and forget to sync. Then I start my computer and I read and annotate a document. Computer syncs. Then I read my book again in the subway and when I get home I sync. I loose either the data of what I did at work or of what I did in the subway.

### To reproduce|重现步骤

Steps to reproduce : annotate on device 1 without internet connexion (outdoor). Then annotate on device 2 with internet connection. Device 2 changes will sync. Then put device 1 on internet connexion again. Either you will losse changes on device 2 or changes on device 1. There is no sync.

Workaround : make sure you sync your data before and after every usage, particularly offline, and before using another device.

### Expected behavior|预期行为

- In the current state, it shouldn't be called "sync" but "upload/download" and user should be warned that no sync is happening and he has to choose only upload or download.
- There is a possibility to revert to a previous backup and export it. But it exports all of the database. If it was possible to export/import the data of one single book, then dataloss would be avoided with a little export/import work.
- The final behavior would be a real sync.

### Screenshots|截图

_No response_

### Platform (please complete the following information)|平台（请填写以下信息）

All platforms

### Additional context|其他信息

_No response_