Add progress bar during file upload

### Description

When uploading large files, the process can take some time. Therefore, it would be preferable to add a progress bar instead of the current dots/loading icon, to give users a clear indication of how much of the file has been uploaded or how much time remains.

When uploading multiple large files simultaneously, the current loading indicators (dots) do not reflect the status of each file individually. Instead, the indicators switch to 'Ferdig lastet' only after all file uploads are complete. This process can take several minutes when handling many large files.

### Additional Information

_No response_

Start by locating the file-upload UI and its current dots/loading indicator. Trace how upload status is reported for one file and for simultaneous files, then define done as showing accurate, independent progress for each file until completion.

Written by the indexing model from the issue text.