Show participant count on spotlight tile

In group calls, to help people stay aware of how many participants are in the call even while the grid view is not visible, we should show a participant count on the spotlight tile:

https://www.figma.com/design/aOEkaJtaBmPy058V7uoqVr/Element-Call-Updates---2026?node-id=20-10980&t=ae2fanXuOQlg58sd-4

https://www.figma.com/design/pDlJZGBsri47FNTXMnEdXB/Compound-Android-Templates?node-id=2255-17676&t=OeMFR378cIbhfNBq-4

The participant count should **not** be visible in the grid or picture-in-picture layouts.

Start by reviewing the two linked Figma designs and locating the spotlight, grid, and picture-in-picture layout components in the TypeScript frontend. Add the participant count to the spotlight tile only, then verify it is absent from grid and picture-in-picture layouts and matches the designs.

Written by the indexing model from the issue text.