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

Start with ROADMAP.md → Control panel, inspect the existing REST API, and read related issues #391 and #412. The RFC spike must clarify hosting and authentication before implementation is bounded; done should be an agreed Phase 1 read-only scope with defined views, trace links, and live TaskEvents behavior.

Written by the indexing model from the issue text.