Atrium 0.103.4 — A long conversation of heavy turns opens again

Patch release (hotfix). The Convex functions and the frontend change; the bridge and the
schema do not, and nothing needs migrating. Deploy the Convex functions, then the frontend.
Either half works with the other half of 0.103.3 during the rollout.

• A long conversation no longer fails to open. The conversation view loads the most
  recent 200 messages with everything they carry (tool results, rows, files). On a long
  conversation of heavy turns this added up past the backend's per-request read limit
  (16 MiB): the request failed, the page showed its error screen, and retrying failed the
  same way. Nothing was lost — only the read failed. The view now stops reading
  before that limit: it shows the most recent messages that fit, complete, and leaves the
  older ones out, exactly as it already did past 200 messages. Your messages and their
  content stay stored; only how many of them the page loads at once changes.
• A single turn too heavy to load on its own still opens. When the most recent message
  alone exceeds the budget, it is shown with its text but without its attachments, tool
  cards and rows, rather than failing the page.
• The export says how many messages it holds. The "export limited to the most recent
  messages" note now gives the number the export contains. For a conversation past 200
  messages it reads exactly as before; for a shorter window cut by the read limit it no
  longer claims 200.
• Branching and the diagnostics hold the same bound. Branching such a conversation
  into a new chat copies the most recent history that fits (reported as truncated, as a
  branch past 200 messages already was) instead of failing; the branch point is always
  copied. The chat-state and diagnose_chat diagnostics read the same bounded window
  and report windowTruncated (and partsOmitted on a turn shown without its parts).

Rollback: redeploy the 0.103.3 Convex functions and frontend. No data changes in
either direction; a heavy conversation fails to open again after a rollback.