fix(flow-chat): smooth action lifecycle transitions

[bobleer]

Summary

• Keep just-completed FlowChat actions visible while they remain the visual tail, then compact them when newer conversation content arrives.
• Preserve stable explore-group and tool-card render identity across running-to-completed transitions.
• Harden collapse reversal, pre-paint automatic state changes, and reduce expensive card-level transition, blur, breathing-opacity, and will-change work.
• Add regression coverage and document the FlowChat scroll-height stability contract.

Type and Areas

Type:

UI/UX, regression fix, test, docs.

Areas:

Web UI, FlowChat virtualization, tool cards, subagent projection.

Motivation / Impact

Tool completion could simultaneously change virtual item shape, hide the settled card, and collapse its containing group. Users saw the action flash and disappear, while animated automatic height changes made the virtualized scroll anchor chase the layout. Completed tail actions now remain readable until superseded, automatic compaction is applied before paint in one frame, and manual toggles retain smooth animation.

Verification

• pnpm --dir src/web-ui run test:run with 7 focused FlowChat files: 89 tests passed.
• pnpm run type-check:web
• pnpm run lint:web
• git diff upstream/main...HEAD --check
• Manual in-app Browser preview: verified expand, collapse, rapid reversal, settled will-change cleanup, and card transition/backdrop computed styles.

Reviewer Notes

Automatic compaction intentionally does not animate because Virtuoso must observe one stable height change. User-initiated expand and collapse actions continue to animate. No user-facing strings or locale resources changed.

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.