feat(ui): operator control panel web UI

**Context:** ROADMAP.md → Control panel

---

## Component

Other

## Describe the feature

**Web UI** for operators: task list, task detail (logs/traces/events), cancel, metrics dashboards, cost attribution. Task detail shows manager-style progress alongside raw events/traces.

## Use case

CLI-only ops do not scale for team visibility. Stakeholders need a browser-based view without AWS console access.

## Proposed solution

1. SPA (or SSR) authenticated via Cognito; calls existing REST API.
2. Views: tasks, task detail, approvals queue, cost dashboard hooks (#237, #215).
3. Embed trace download links; live poll `TaskEvents`.
4. Host on CloudFront + S3 or App Runner—TBD in RFC spike.
5. Phase 1 read-only; Phase 2 cancel/approve actions.

## Other information

- Related: #391 (operator session debug), #412 (stats), smart progress draft.
- May warrant follow-up RFC for hosting/auth architecture.

- [ ] This might be a breaking change

ROADMAP.md → Control panel から始め、既存の REST API を調査し、関連する issue #391 と #412 を読みます。実装の範囲を定める前に、RFC spike でホスティングと認証を明確にする必要があります。完了条件は、定義されたビュー、trace link、およびライブの TaskEvents の動作を含む、合意済みの Phase 1 の読み取り専用スコープです。

索引モデルが issue の本文から書いたものです。