remember notification settings on open conversations between leave/join

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
latest stable - 28.x

Is your feature request related to a problem? Please describe.
There's a bunch of open chats on our server. I join and leave regularly. But some are team chats that have regular calls, so I have to disable the notification for calls, otherwise I go crazy. But after I leave a room and re-join, I have to disable this notification again... it doesn't store it.

Describe the solution you'd like
Talk should remember my settings per room, even when I exit the room and join again.

Describe alternatives you've considered
A global default would be nice, but it doesn't fully replace this.

Start by tracing Nextcloud Talk's per-room notification settings flow and how a user's room membership is handled when they leave and rejoin. The feature is done when call-notification preferences are retained for each room across that membership cycle, while a global default remains only an alternative.