[BUG] Artifact iframe missing microphone in allow attribute — getUserMedia always NotAllowedError

The claude.ai artifact viewer iframe doesn't delegate microphone, so getUserMedia({audio:true}) in a published artifact fails with NotAllowedError before any prompt. Browser settings can't override it (they govern the top-level origin, not the frame).

Repro: publish an artifact with a button calling getUserMedia({audio:true}), open on claude.ai, tap → immediate NotAllowedError, no prompt.

Fix: add microphone to the iframe's allow attribute so the browser can prompt normally.

Same mechanism as #87905 (Chrome side panel); this one is the artifact viewer.