Update thread summary tile

[germain-gg]

## Design

So that it matches new UI explorations

Figma file: Flow – Viewing a replying to a thread (figure 01a)

Requirements

• Display the number of replies in the thread, counting only the events that are loaded via the SyncApi unless the thread's info aggregation API has landed on Synapse
• Display a 1 line clamped preview of the message using the same logic that is used for the room tile message preview #19398
• Clicking on the thread info should highlight the previewed message #19404
• The thread summary should still be displayed even if the root message has been deleted

Out of scope

• The notification bell