Fix sent message editing interaction

[SuperstructureJH]

Summary

• edit only the latest text-only sent user message inline, matching the Codex interaction boundary
• keep older user messages and steering rows non-editable while preserving their existing actions
• submit the edited prompt only after confirmation, then replace the visible conversation instead of leaving a second sidebar session
• preserve workspace, title, and agent preset; clean up temporary revisions on failure

Supersedes #21, which was closed before this interaction correction.

Verification

• npm ci (patch applies from a clean dependency install)
• npm test (42 tests passed)
• npm run typecheck
• npm run build
• node --check node_modules/@deepseek-ai/dsh-client-ui-conversation/lib/client.js
• npm run package:mac:arm64