BUG: Registration form not working on mobile properly and missing progress indicator

## 🐛 Bug Report

Registration pages fail to render properly on mobile views, show error messages that are not easy to understand for non-programmers and lacks a way to show how many sections a participant must fill in to complete the registration form

### Registration form doesn't render properly on mobile

### Error messages are hard to understand for non-programmers

### It doesn't show how many more sections a participant have left to complete the registration process

No files, tests, or entry points are named. Start by locating the registration pages and their mobile layout, validation messages, and section navigation. Done means the pages render correctly on mobile, errors are understandable to non-programmers, and participants can see how many sections remain.

Scritto dal modello di indicizzazione a partire dal testo della issue.