fix(web-ui): stabilize turn list navigation

[snvtac]

Summary

• Keep the Flow Chat turn list open after selecting a non-current turn until the selected turn is actually reflected as the current header turn.
• Queue header turn-list navigation when VirtualMessageList.pinTurnToTop is temporarily unavailable, then retry when virtual items change.
• Add regression coverage for the real header close behavior and the container retry path.

Fixes #1281

Type and Areas

Type:

bug fix / regression fix / UI/UX / test

Areas:

web UI, Flow Chat

Motivation / Impact

Long Flow Chat sessions could close the header turn list after clicking a turn even when the virtual list had not accepted the navigation request yet. Users saw the menu disappear while the chat stayed in place.

This change keeps the menu open for unaccepted non-current turn selections, retries the container-level turn pin when the virtualized items update, and only lets the header advance once the virtual list accepts the request.

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/FlowChatHeader.test.tsx src/flow_chat/components/modern/ModernFlowChatContainer.history-state.test.tsx
• pnpm run type-check:web
• git diff --check
• Two independent agent reviews of the final diff reported no blocker findings.

Reviewer Notes

The fix is scoped to header turn-list navigation. It does not add side-panel history navigation or change VirtualMessageList pin semantics.

AI-assisted: yes. Testing level: focused automated regression tests plus Web UI type-check.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.