fix(ui): RunHistoryStrip setState-in-effect lint failure blocks frontend lint gate

[w7-mgfcode]

Problem

pnpm lint (the frontend eslint . gate) fails on dev due to a pre-existing
react-hooks/set-state-in-effect error in
frontend/src/components/demo/RunHistoryStrip.tsx (the effect calls
setItems/setLastSummary synchronously in its body). eslint-plugin-react-hooks@7
(pinned in the lockfile) promotes this to an error, so the whole-repo lint gate
is red.

This is unrelated to any feature work — it blocks the frontend lint gate for
every branch. Fixing it in isolation keeps feature PRs green.

Scope

• frontend/src/components/demo/RunHistoryStrip.tsx ONLY.
• Refactor the run-history append so it no longer calls setState synchronously
  inside the effect body (derive/guard appropriately), preserving the existing
  HISTORY_CAP + persistence behavior.

Acceptance

• pnpm lint reports 0 errors for this file.
• pnpm tsc --noEmit clean; pnpm test --run green.

[w7-mgfcode]

Fixed in 5a49fe3 (avoid setState-in-effect in RunHistoryStrip), merged via #358; shipped in v0.2.21. Closing.