fix(web): preserve transcripts and surface run failures

[iancaoo]

Summary

• prevent same-session conversation refreshes from replacing the live transcript with an older persisted snapshot
• surface persisted and live run failures above the chat composer when the task console is closed

Problem

A background conversation restore could race with a follow-up query and temporarily remove an earlier assistant response until a newer snapshot arrived. Separately, terminal run errors were visible only in the task console, making timed-out or failed runs appear stuck when the console was closed.

Changes

• only allow message replacement during an intentional thread restore or when the agent transcript is empty
• keep same-thread background refreshes for checkpoints, usage, branches, and run state without overwriting existing messages
• pass the current live-run error into the chat input bindings and render it through the existing composer error alert
• add regression coverage for same-thread stale snapshot replacement

Verification

• npm --workspace @datafoundry/web run test -- src/app/data-tasks/__tests__/conversation-restore.test.ts src/app/data-tasks/__tests__/run-error-message.test.ts
• npm run typecheck
• npm run build:web

Closes #102

[cyber-luoyang]

lgtm

[wangjk9527]

LGTM