WebUI Smoke CLI — installation verification e2e runner

## Overview

A dedicated CLI tool (`backend.ai-webui-smoke-cli`) that lets Field-Ops engineers verify Backend.AI WebUI functionality immediately after on-prem / air-gapped installs, without requiring developer tooling (no pnpm, no monorepo checkout, no `.env.playwright` editing).

The CLI re-uses the existing `e2e/` Playwright assets by selecting specs tagged `@smoke`, runs them against a customer-provided endpoint, and emits an HTML+JSON report directory that can be handed off as a deliverable.

## Goals

- Single-binary (Phase 2) / single-package (MVP) execution
- Air-gap operation — no external network during test run
- Single-account assumption with auto role detection (admin / user / monitor)
- Self-contained HTML report + machine-readable `summary.json`
- App-launcher coverage as the headline (most common failure mode at customer sites)

## Out of scope

- Visual regression testing
- Backend.AI backend (Manager/Agent) health checks — separate tooling owns that
- Load / performance testing

## Phasing

- **MVP**: FR-A (tag convention) → FR-B (workspace scaffold) → FR-C (runner). Sufficient for staging usage.
- **Phase 2**: FR-D through FR-J. Required before customer-site delivery.

## Spec

`.specs/{this-epic-key}-webui-smoke-cli/spec.md`

## Related plan

`docs/plans/webui-smoke-cli.md` (in the worktree).

JIRA Issue: FR-2871