Surface Slack install failures in the UI

When someone tries to install the Hive Slack app and the OAuth handshake goes wrong, Hive now tells you why instead of failing silently. Specific cases are now explained in the UI: an expired install link, a cancelled install (access_denied), other rejections from Slack (with the underlying error text), and the case where Slack returns no authorization code. Failures are rendered as dismissible alerts in a new flash stack pinned to the top of the page, so admins can immediately see what went wrong and retry with the correct action.