fix(web): a person's participant address is their mailbox address (CL-8581)

[TheGreatAxios]

Summary

• A person participant's roster address was principal.email ?? principal.refId — neither is a routable mailbox address, so agents copying a person on a handoff (per the roster instruction) sent to an address the hub drops, and the person's own turn never matched itself for avatar resolution.

Changes

• listRoomParticipants now takes the room tenant's own domain (already fetched by every caller) and builds a person's address as <refId>@<domain> — the same address the hub builds for mailbox delivery.
• Added sameAddress, a case-insensitive whole-address comparison, and used it everywhere a turn's address is matched against a participant's (resolveParticipantName, resolveAvatarName, the room page's avatar-kind lookup) — the mailbox lowercases local parts on the wire, so a header from and an envelope from for the same sender can differ in case.
• Callers (workbench-room-page.tsx, workbench-create.ts) pass the domain they already have in scope; no new fetch was added.

Testing

• bun test apps/web/src/chat/threads-api.test.ts apps/web/src/chat/room-roster.test.ts
• bun run check

[linear-code]

CL-8581