[FEAT] Happy Path: improve the rendering of the overlay on the start event

[tbouffard]

Is your feature request related to a problem? Please describe. The overlay on the start event looks strange. It is not centered and it is not very visible.

Describe the solution you'd like Proposals to be discussed

• change the position of the overlay
• use white color for the overlay stroke.
• OR, use a tippy.js popover instead. This is what is done in https://github.com/process-analytics/bonita-day-demo-2023/issues/43

Note: a first improvement was done in #79