The indefinite progress bar is confusing before firmware updates

## Summary
When updating Ping1D firmware, there's an indefinite progress bar that scrolls until an actual firmware update gets confirmed and starts. This can feel like it's doing some kind of loading before the flashing becomes possible, and can be extra confusing when doing a manual update because once you select the firmware from the filesystem and return to Ping Viewer then there's a progress bar looking thing there, and it feels like the firmware might be flashing, but it's not until the "Firmware Update" button gets clicked.

Ideally
1. the progress bar would be hidden until the firmware flashing actually starts, and
2. we may want to automatically start a firmware flash once a file has been selected, especially for the manual updates
- this could go either way - there may be benefits to requiring the button press for confirmation too 🤷‍♂️

## Additional information
Current interface, before flashing has started:

Raised in [this PR comment](https://github.com/bluerobotics/ping-viewer/pull/1081#pullrequestreview-2110984746)

Start by tracing the Ping1D firmware update UI around manual filesystem selection and the "Firmware Update" button. Confirm when flashing actually begins and how the indefinite progress bar is activated. Done means the bar is hidden before flashing, with the issue resolved on whether selecting a manual firmware file should start flashing automatically.

Written by the indexing model from the issue text.