Add more noticeable markers for the running cell/notebook

[secsilm]

This is a great project, and I think I will try it out at work. Currently, I feel there are two points that could be optimized:

1. If a notebook contains many cells, it might not be easy to identify which cell is running when switching back from another page.
2. When switching to another tab, it currently seems impossible to determine from the notebook tab's title whether it is still running. You could refer to the approach used in Jupyter Notebook, which has a special indicator.