Allow frontend chat for resolved non-user audience principals

## Problem
Frontend Agent Chat currently hides the FAB for anonymous visitors because it depends on `frontend_agent_chat_list_accessible_agents()`, while `frontend_agent_chat_user_can_see()` explicitly bails when `! is_user_logged_in()`. That is correct for public internet defaults, but it prevents trusted non-user audiences from using selected frontend agents.

## Concrete scenario
A WP Cloud runtime can be private/proxied at the edge. Public internet receives a 403 before WordPress, while requests that reach WordPress can represent an internal audience. The FAB should be able to appear for that audience once Agents API/Data Machine resolve and authorize an audience principal.

## Proposed shape
- Replace the hard logged-in-user visibility gate with a generic "current principal can access at viewer/operator role" check.
- Continue hiding the FAB when no principal or accessible agent exists.
- Avoid site-specific bypass filters; consume the generic Agents API/Data Machine audience access contract.

## Acceptance criteria
- Logged-in user behavior is unchanged.
- Anonymous public visitors still see nothing unless a host resolves an authorized principal/audience.
- A trusted audience principal can list accessible agents and use the chat widget.
- The widget can require operator access when chat execution needs operator-level tools.

Start by tracing frontend_agent_chat_list_accessible_agents() and frontend_agent_chat_user_can_see(), then inspect the Agents API/Data Machine audience access contract they consume. Verify that logged-in behavior remains unchanged, public anonymous requests remain hidden, and a resolved authorized audience principal can list agents and use the widget with viewer or operator requirements.

Written by the indexing model from the issue text.