ui: Replace 'Current frame' text with a visual progress bar

The "Current frame: x / y" text at the bottom of the sidebar is functional, but a visual progress bar would be a much better way to show the animation's progress.

Task:

Remove the st.write() that displays the current frame text.

In its place, add an st.progress() bar.

The value of the progress bar should be calculated as (current_frame / total_frames).

Ensure the progress bar updates on every step of the animation, starting at 0% and finishing at 100%.