Chat relay follow-ups

* Follow up to https://github.com/nextcloud/talk-ios/pull/2404

- [ ] Send read marker only for visible messages, while still updating the locally stored one
- [x] Handle `lastCommonRead` after https://github.com/nextcloud/spreed/pull/18433 & https://github.com/nextcloud/spreed/pull/18488
- [x] https://github.com/nextcloud/talk-ios/issues/2580 // https://github.com/nextcloud/talk-ios/pull/2611
- [ ] Check if chat relay is only activated after we joined external signaling (seems it's currently activating it before we joined, only in NC)
- [x] https://github.com/nextcloud/talk-ios/issues/2594 // https://github.com/nextcloud/talk-ios/pull/2596
- [x] https://github.com/nextcloud/spreed/pull/18572 // https://github.com/nextcloud/talk-ios/pull/2593

Start by reviewing the follow-up context in PR 2404 and the remaining unchecked checklist items. Investigate visible-message read markers and whether chat relay activates before joining external signaling; the work is done when both items are resolved and the checklist reflects the final behavior.

Written by the indexing model from the issue text.