fix(web): claimant avatars in the live session have no accessible name

[aek676]

In the live session list, the claimant avatars render as plain divs: screen readers only get the initial and the count ("S", "1") with no participant name or context.

Give each avatar an accessible label along the lines of "SecondGuest claimed 1" (aria-label or visually-hidden text) in apps/web/src/features/session/components/ (claimant avatars component).

[pullfrog]

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