You will not lose data from the main server/database if you delete entries from your mobile device, provided that:
- The entries were uploaded successfully without any errors.
- You have double-checked that the entries are all uploaded on the server (in the web interface).
- (Optional, but extra safe!) You have downloaded a CSV backup from the server to keep a local copy.
If you have confirmed your data is on the server, you can safely delete the entries from your phone. The server and the app are independent once the upload is complete and confirmed.