Add progress bar to clock page.

The timer would benefit from a visual component that acts as a progress bar, showing the breaks taken so far and the upcoming point where the next break will be rolled.

The bar would grow initially, and would most likely scroll horizontally for longer sessions. Maybe we could even save the completed bar as a thumbnail for the session when completed.

This most likely can be implemented in Svelte itself. If any library can be used, it would be best if it is lightweight, unless there's a really good use case for it.