fix(web): person turns show the signed-in name on the avatar (CL-8508)

[TheGreatAxios]

Summary

• A room's own message turns showed a "Y" avatar (from the internal authorName: "You" transcript label) while the Participants panel already showed the person's real display name for the same principal.

Changes

• apps/web/src/chat/threads-api.ts: add resolveAvatarName, which prefers the matching room participant's real name (including the person's own) for avatar purposes, falling back to the existing resolveParticipantName behavior — the transcript label itself still reads "You" where shown.
• apps/web/src/pages/workbench-room-page.tsx: RoomMessageRow's avatar now uses resolveAvatarName instead of the raw "You" label, sourced from the room participants list already fetched for the page (no new fetch added).

Testing

• bun test src/chat/threads-api.test.ts
• bun run check

[linear-code]

CL-8508