feat(ui): operator control panel web UI

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

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## 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

从 ROADMAP.md → Control panel 开始，检查现有的 REST API，并阅读相关 issue #391 和 #412。在限定实现范围之前，RFC spike 必须明确 hosting 和 authentication；完成标准是达成一致的 Phase 1 只读范围，其中包含已定义的视图、trace link 以及实时 TaskEvents 行为。

由索引模型根据 Issue 内容生成。