Attendees should see event details even when event is set to “Show as busy when shared”

Is your feature request related to a problem? Please describe.

I share my personal calendar with multiple users so they can see my availability, but I do not want them to see event details.
For each private event I set the visibility to “When shared: show only busy” (i.e. free/busy only).

However, when I invite one of those users to that same event, they receive the full details by email (ICS/invitation),
but inside Nextcloud Calendar (web UI) they still see the event as “Busy” with no title/description/location.

This is frustrating because:

• non-invited shared users should only see free/busy ✅ (privacy)
• an invited attendee should be able to see the full event details ✅ (collaboration)

Describe the solution you'd like

When a user is an ATTENDEE of an event, Nextcloud Calendar should display full details to that attendee,
even if the event visibility is set to “When shared: show only busy” for shared calendars.

In other words:

• Shared users (not attendees): show only free/busy
• Invited attendees: show full details

If this is not possible due to iCalendar CLASS:CONFIDENTIAL semantics, an alternative could be:

• Add a separate visibility option such as “Show only busy when shared, but show full details to attendees”
  or
• Treat attendees as privileged viewers for confidential events.

Describe alternatives you've considered

• Not sharing the calendar at all (not acceptable, people need availability).
• Duplicating events: one confidential for the shared calendar + one public event for the attendee (tedious and error-prone).
• Maintaining separate calendars and moving events around (also tedious and easy to forget).

Steps to reproduce

1. User A creates a calendar and shares it with User B as read-only (or any share level where B should not see details).
2. User A creates an event and sets visibility to “When shared: show only busy”.
3. User A adds User B as an attendee to that event.
4. User B receives an email invitation with full details.
5. In Nextcloud Calendar UI, User B opens the event.

Expected behavior

User B (attendee) can see the full event details (title, description, location, etc.) in Nextcloud Calendar.

Actual behavior

User B still sees only “Busy” and cannot see event details.

Environment

• Nextcloud Server: 32.0.3 (self-hosted)
• Calendar app: 6.1.4
• OS: Debian stable (server)
• Web server: (nginx/apache)

Additional context

This is a common use case in organizations: share a personal calendar for availability while keeping details private,
but still be able to invite someone to a meeting and have them see full details.

Start in the Nextcloud Calendar web UI path that renders shared events and trace how ATTENDEE status and “When shared: show only busy” are evaluated. Check the iCalendar CLASS:CONFIDENTIAL semantics against the reproduction steps. Done means non-attendees still see only busy while attendees see the title, description, location, and other event details.