Add Dutch day names to event dates

[Copilot]

Event dates were displayed without the day name, making it harder to quickly parse when an event falls. The requested format: zaterdag 13-05-2026 19:30 tot zondag 14-05-2026 21:30.

Changes

• config/locales/nl.yml — Two new named time formats using %A (localized day name):
  • with_day_name: "%A %d-%m-%Y %H:%M" — standalone datetime
  • with_day_name_prefixed: "Op %A %d-%m-%Y om %H:%M" — list view with Dutch prepositions
• events/show.html.erb — Start/end time use l(date, format: :with_day_name)
• events/_list.html.erb — Uses l(date, format: :with_day_name_prefixed)
• external_signups/ — Public signup pages get the same treatment
• Mailer templates — All event email templates updated to include the day name

All changes use Rails' l() helper so %A resolves via the existing date.day_names in nl.yml — no hardcoded Dutch strings in views.