[Enhancement]: Add a Gateway release-channel dropdown to managed setup

Summary

Add a user-visible release-channel selector to managed Gateway setup.

The current wizard presents one preselected Gateway version without showing or allowing the user to choose the release policy behind it.

Current behavior

On 2026-09-05, the released Hub's setup summary displayed:

Official Gateway 2026.6.34; validated for protocol v4 and fetched over HTTPS from openclaw.ai.

Current main (305bb4ef5334ded9f8541aca19c66ba2c4b08754) still defines 2026.6.34 as RecommendedVersion and exposes only recommended, fallback, or exact through configuration.
There is no release-channel control in the managed setup UI.

At review time, npm reported:

{
  "extended-stable": "2026.6.34",
  "beta": "2026.9.1",
  "latest": "2026.9.1"
}

Proposed user experience

Add a release-channel dropdown to managed Gateway setup with these choices:

• Stable, mapped to npm latest, as the recommended default.
• Beta, mapped to npm beta.
• Extended stable, mapped to npm extended-stable.
• Exact version as an advanced option, if exact-version installs remain supported.

The review screen should show both the selected channel and the exact version it resolves to before installation.

Expected behavior

1. The user can choose the desired Gateway release channel during managed setup.
2. The setup review clearly identifies the chosen channel and resolved package version.
3. The installed configuration preserves enough information to distinguish the user's selected policy from the exact installed version.
4. Existing installations are not silently moved to a different channel.

Relationship to existing work

Issue #1000 tracks keeping the Windows Companion aligned with current OpenClaw stable and beta releases.

PR #1312 changes new managed installs to npm latest by default and adds backend support for latest, stable, extended-stable, beta, dev, next, and exact versions.
That PR would address the stale hidden default and much of the underlying selector model, but it does not add a user-facing release-channel dropdown.

This request is therefore limited to exposing that choice in the managed setup UI and making the selected policy visible to the user.

Acceptance criteria

• Managed Gateway setup includes a release-channel selector.
• Stable is the clearly identified recommended default.
• Beta and extended-stable are available choices.
• Exact version is available as an advanced choice if the backend continues to support it.
• The review screen displays the selected channel and resolved exact version.
• Existing installations retain their current channel or exact-version intent unless the user changes it.
• Each supported UI path has functional test coverage.

Reproduction scope

The displayed 2026.6.34 behavior was reproduced with released Hub v2026.7.1-4 on a clean Windows VM.
The absence of a user-facing channel selector was also confirmed by source review of current main.

Start by tracing the managed Gateway setup UI and reviewing PR #1312 for the existing channel and exact-version backend support. Add the selector and review-state display for Stable, Beta, Extended stable, and any supported exact-version path, then add functional coverage for each supported UI path and verify existing installations retain their intent.