Split global search into Rooms and Messages tabs. The Messages tab lists
matching events with the room's name and avatar, a sender-prefixed preview,
rich media (image/video/audio/file) previews, and a loading indicator, then
paginates as you scroll. Queries are debounced and the previous one is
superseded so late results can't clobber a newer search.
Wrap the SDK's SearchService in a SearchServiceProxy that publishes typed
message results and paginates on demand, expose it on the ClientProxy, and
enable the search index store on the session. Also opens up the timeline item
factory's message content builder so the proxy can reuse it.
Stefan Ceriu