feat: unify channel and thread display titles

[MartinCupela]

🎯 Goal

Closes REACT-871
Closes REACT-829

Depends on GetStream/stream-chat-js#1699

Remove ThreadHeaderMain component as it is not necessary anymore (introduced temporarily due to different designs)

Hook useChannelPreviewInfo now generates data by subscribing to channel displayStore and membersStore; derive groupChannelDisplayInfo from membersStore (no getGroupChannelDisplayInfo).
GroupAvatar / ChannelAvatar: rename prop members → displayMembers.
ThreadHeader use channel display from store; fallback without thread to channel display then parent message author (name only).

• Fix bug when group avatar for channel with more than 4 members was still showing variant for channels with 4 members.
• Fix bug of not showing reply count in threads in 'threads' view in the list head - ThreadStart

BREAKING CHANGE: useChannelPreviewInfo now always returns a defined groupChannelDisplayInfo (object with members and overflowCount). For 1:1 or ≤2 members it returns { members: [], overflowCount: undefined }. Code that treated groupChannelDisplayInfo == null as “not a group” should use e.g. groupChannelDisplayInfo.members.length >= 2 instead.