Maity: Board

Review PR #156 (web: chat history polish part 1)

tsk-0ezsjbpsjc3a cancelled chore

Independent review of https://github.com/oliver-zehentleitner/maity/pull/156 (agent D): day separators, grouping, New-messages line, jump to newest, agent picture, empty state. Check timezone/day boundaries, unread line correctness with paging, a11y (separators announced sensibly, focus), 360px, XSS. Deliver review_result to oz-agent-a.