Make chat messages feel collaborative

[rogerchappel]

Summary

• Rework chat messages into a Slack-style row with avatar on the left and author metadata above the content.
• Move message timestamps into the author line beside the display name.
• Pass user and selected-agent identity into main chat and thread messages so labels are no longer just AI/YOU.

Verification

• pnpm typecheck
• pnpm lint:check (passes with existing warnings)
• git diff --check
• pre-push hook: pnpm typecheck && pnpm build

Notes

• Build completed with an existing Turbopack tracing warning related to next.config.ts/import tracing.