Banner groups — much nicer to use

- Visual date picker with flatpickr in your language (no more typing in YYYY-MM-DD).                                                                

 - Smart timeline: green "connected" indicator between banners, yellow "gap" notice, red "overlap" warning.

 - Auto-fill: when creating a new banner, the start time is automatically set to +1 min after the previous one.                                                         

 - "Create next banner" CTA after your last item — just click through.                                                                     

 - Banners are automatically sorted by start date; no more manual ordering.                                                               

 - Group name, end date, and planning note can be edited directly from the overview.                                                            

 - Active groups collapse into accordion rows for a cleaner overview.                                                                   

 - Banner expiry reminders now come at 7/4/1 days before end (instead of 14/7/1).