fix(web-ui): stabilize FlowChat history paging

[wsp1911]

Summary

Fixes #1537

FlowChat history paging no longer replaces the active scroller during upward navigation. The existing static history viewport stays mounted, expands before prepend, and restores the visible user-message anchor across both expansion and older-history insertion.

The loading sentinel is shown only while older history remains available. It is removed when isPartial becomes false, so the first turn aligns directly with the chat header.

Type and Areas

Type:

Regression fix / UI/UX / test

Areas:

Web UI / FlowChat / virtual message list

Motivation / Impact

Previously, scrolling toward the oldest loaded turn could insert a large estimated spacer, briefly blank the viewport, and restore the user to the wrong position. An intermediate attempt to switch from the static list to Virtuoso during the scroll transaction could also return the viewport to the latest turn.

This change keeps the scroller stable, preserves a semantic message anchor, loads older turns near the actual boundary, and removes the loading placeholder once the first turn is reached.

Verification

• pnpm exec vitest run src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx src/flow_chat/components/modern/VirtualMessageList.layout.test.ts --pool=threads --maxWorkers=1 --no-file-parallelism from src/web-ui
  • 63 tests passed
• pnpm run type-check:web
  • Passed
• pnpm exec eslint src/flow_chat/components/modern/VirtualMessageList.tsx --no-warn-ignored from src/web-ui
  • Passed
• git diff --check
  • Passed
• Manual reproduction confirmed the main history paging behavior; the final first-turn sentinel removal is covered by the focused regression test.

Reviewer Notes

• The static scroller remains active during the paging transaction; no renderer switch occurs while the user is scrolling.
• The semantic anchor is a rendered user-message element and is restored in a layout effect before the next paint.
• The loading sentinel reserves a fixed height while partial history remains, preventing layout movement when its visibility changes.

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.