Allow selecting an existing one-to-one Talk conversation for calendar events

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

When creating a calendar event with another Nextcloud user, I cannot select the existing one-to-one Talk conversation with that user.

This is inconsistent with the workflow in Talk: when scheduling an event from an existing one-to-one Talk conversation, that conversation can be associated with the event.

When creating the same event from Calendar, however, the existing one-to-one conversation is not available under "Add Talk conversation" → "Select existing conversation". This often results in creating an additional Talk room just for the event, even though a suitable conversation between the two users already exists.

Describe the solution you'd like

When a calendar event has exactly one other internal Nextcloud user as attendee, Calendar should allow selecting the existing one-to-one Talk conversation between the organizer and that attendee.

Ideally, this matching conversation could be suggested or shown prominently in the list of available Talk conversations.

Other one-to-one conversations that do not match the attendee should not be shown.

Describe alternatives you've considered

Currently I can:

• create/schedule the event from the existing Talk conversation instead of Calendar, or
• create/use a separate group Talk conversation for the calendar event.

Both are workarounds. The first requires starting the workflow in Talk, while the second creates an unnecessary additional conversation for a meeting between two users.

Additional context

The Calendar Talk conversation picker currently appears to explicitly restrict selectable conversations to group (type === 2) and public (type === 3) conversations in AddTalkModal.vue, which excludes one-to-one conversations (type === 1).

The requested behavior is not to show all private one-to-one conversations. It should only make the one-to-one conversation available when it corresponds to the single other Nextcloud user invited to the event.

This would also make the Calendar → Talk workflow consistent with the existing Talk → Calendar workflow.

Start with AddTalkModal.vue, especially the existing filtering for Talk conversation types, and trace how calendar attendees are available when selecting a conversation. Verify the Talk → Calendar behavior for one-to-one conversations. Done means an event with exactly one other internal attendee can select the matching one-to-one conversation, while unrelated private conversations remain hidden.