Feature Request: Import participant list from CSV, XLS, or JSON

[Spaccesi]

Summary

Currently participants must be added manually one by one. Supporting bulk import from files would significantly improve usability for large events.

Possible Solution

• Add an "Import" button in the participants screen.
• Support .csv, .xls/.xlsx, and .json formats.
• Map the first column / a name field to the participant name.
• Show a preview before confirming the import.

Acceptance Criteria

• Users can import from .csv, .xlsx, and .json.
• Invalid rows are flagged and skipped with a summary shown.
• A preview of participants is shown before final import.
• Duplicate detection applies during import.