Deliverable: Give teams control to disable FlowFuse Expert

## Problem

Customers need to trust that FlowFuse Expert respects their data governance requirements. Today, there is no way for a team owner to disable FlowFuse Expert — the only options are "don't use it" or manually removing the assistant plugin via Node-RED's palette manager. Neither is acceptable for organizations with strict policies around AI tooling.

This is a trust and adoption blocker: teams that cannot explicitly opt out are less likely to adopt FlowFuse at all, even when FlowFuse Expert would benefit other teams in the same organization.

## Goal

Provide a first-class, team-level setting that lets team owners disable FlowFuse Expert for their team — with clear communication about what the feature does, what data flows when it's used (and that nothing flows when it's not), and a path to re-enable it.

## Key Principles

- **Transparency over restriction:** Even when disabled, the UI should inform users about what FlowFuse Expert is and why it exists — not hide it entirely
- **No data without action:** Regardless of the toggle state, no data is externalized to AI providers unless a user actively sends a prompt. The setting is an organizational policy control, not a data leak fix
- **Separate Expert from Assistant:** The `@flowfuse/nr-assistant` package provides functionality beyond FlowFuse Expert (e.g., tab title sync, future platform integrations). Disabling Expert must not break non-Expert assistant features

## Context

- **Slack thread:** https://flowfuse.slack.com/archives/C09QS57R8AD/p1775055092055309
- **Related PR:** https://github.com/FlowFuse/flowfuse/pull/7007 (example of assistant functionality that should work independently of Expert opt-out)