List Slack channels for the channel picker

Serves the channel picker of the Slack connect flow. Slack's OAuth installs the bot into a workspace, not a channel, so after the redirect the caller lists the workspace's channels here and finalizes one with POST /v1/connect/slack.

Served by a dedicated route that shadows GET /v1/connect/{platform} for slack.

Parameters

Send exactly one of:

• pendingDataToken: first connect. The nonce from the OAuth redirect, bound to the same profileId.
• accountId: add another channel to a workspace already connected. The existing Slack account's workspace token is reused, no re-OAuth.

With neither, the endpoint behaves like GET /v1/connect/{platform} and returns authUrl and state to start the OAuth flow.

Channels

Channels are read live from Slack (conversations.list, public and private, archived excluded, up to 2,000).

isMember says whether the Zernio bot is already in the channel. A public channel is joined automatically on finalize; a private one must be invited (/invite @Zernio) first.