Relay Baton follow-up: 5 UI bugs from #10832 review

Victor merged PR #10832 (ALL-919 Relay Baton regressions) but flagged 5 follow-ups in #agent-ops 1779202848.276549: (1) input + submit button misaligned on new chat + follow-up composer; (2) no delete-chat affordance — add hover X with confirmation modal (reuse Edges modal); (3) opening past chat: no network call to fetch conversation, content does not load; (4) chat returns markdown but it is not being rendered to markup; (5) Continue in Slack button URL is fabricated — domain is texturehq not jointexture, and app id is empty/invalid → fix or remove pending real Slack app. Branch talos/relay-baton-ui-followups.