Team-level setting to disable FlowFuse Expert functionality

## Context

A customer on Pro Cloud has requested the ability to opt out of FlowFuse Expert. Currently there is no team-level setting to disable this functionality — the only workaround is to not use it (no data is externalized if no prompts are sent) or to manually remove the assistant plugin via Node-RED's palette manager.

We need a proper team-level toggle so team owners can explicitly disable FlowFuse Expert for their team.

**Slack thread:** https://flowfuse.slack.com/archives/C09QS57R8AD/p1775055092055309

## Proposal

### Team Setting

Add a team-level setting (accessible to team owners) that allows disabling FlowFuse Expert functionality for the team. The setting should include:

- A clear description noting that **regardless of whether this is enabled or disabled, if no prompts are sent, no data is externalized to AI providers**
- A link to documentation explaining what FlowFuse Expert is and why enabling it is useful (e.g., agentic creation of Node-RED instances and workflows)

### Behavior When Disabled (proposed)

**In the FlowFuse platform (Flowfuse UI):**
- The FlowFuse Expert button remains visible but is greyed out
- Clicking the button opens the Expert drawer, but instead of the normal chat interface, it shows an **informational view** explaining what the feature is and that it has been disabled by the team admin
- This retains discoverability and provides a path back for teams that later choose to re-enable it

**In the Node-RED immersive editor:**
- FlowFuse Expert is **not available at all** when disabled
- The `@flowfuse/nr-assistant` package itself should NOT be removed, as it provides functionality beyond just FlowFuse Expert
- Only the Expert-specific features should be gated by this setting

### Important Consideration

As noted by Serban in the Slack thread, we need to clearly separate Expert-specific functionality from functionality that is globally available to FlowFuse users. For example, [PR #7007](https://github.com/FlowFuse/flowfuse/pull/7007) (updating the FF title based on the immersive editor's current flow tab) is delivered through the assistant but should work independently of the Expert opt-out. This separation needs to be well-defined as part of this work.