Improve UI for threads and conversations in message list

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

Currently, it is hard to distinguish between single emails and conversations with multiple people. The only indication is the "Re:" in the subject, which can be easy to miss. It's also difficult to distinguish between an unread thread that has a lot of activity vs just one reply.

Describe the solution you'd like

@jancborchardt and I tried to improve this by showing more than one person in the sender list, and adding a count of how many messages in that thread are unread. Gmail, Outlook, Protonmail and Apple Mail all do some form of indicating these.

Some points to note:

• Sender's names are unique and only the 2 most recent senders' names are shown
  For eg.
  Alice sent an email, Bob replied --> Alice and Bob
  Charlie replied --> Bob, Charlie & 1 more
  Alice replied --> Charlie, Alice & 1 more
  Dan replied --> Alice, Dan & 2 more
• Only the most recent sender's avatar is shown
• Unread counter bubble does not need to be shown in case of single emails and threads where all messages have been read
• In case of email threads it shows only the number of unread messages in that thread

Describe alternatives you've considered

No response

Additional context

• Making the timestamp shorter will make this look much better, as @GretaD proposed in https://github.com/nextcloud/nextcloud-vue/issues/2089
• Counter bubble is using --color-primary-light for bg and --color-primary for text

Start with the message list UI and review the design image in this issue, then check the related nextcloud-vue issue #2089 for timestamp guidance. Trace how sender names, avatars, thread state, and unread counts are currently rendered. Done means multi-person threads show the specified recent senders, the latest avatar, and an unread count only when appropriate.