MCP pin control is unreachable on instances below Node-RED 4.0.2 or launcher 2.6.0

### Current Behavior

On an instance that does not qualify for the immersive editor, "Open Editor" navigates to the raw Node-RED URL in a new browser tab. That tab has none of the FlowFuse app around it, so the MCP pin toggle in the platform header is never on screen, along with the rest of the FlowFuse editor UI.

The gate is in [EditorLink.vue:95](https://github.com/FlowFuse/flowfuse/blob/main/frontend/src/pages/instance/components/EditorLink.vue#L95):

```js
// Immersive Editor only available for:
// - Node-RED 4.0.2+
// - Launcher 2.6.0+
```

There is no message anywhere in this path. The user is told by the docs and by their agent to look for a pin control that cannot exist on that instance, and nothing explains why.

Worth being precise about the two halves, because it caused confusion when this first came up: the toggle itself is correctly gated on FlowFuse version and feature flags, not on Node-RED version. The Node-RED version decides whether the user ever sees the header that holds it.

### Expected Behavior

The user is told, at the point of the attempt, that MCP tab pinning needs a newer Node-RED, and what to do about it. Either a banner on the instance or editor page for instances below the threshold, or a disabled state with a reason on the affected control, in the same spirit as [Expert works with instances that are behind on updates](https://github.com/FlowFuse/engineering/issues/324).

### Steps To Reproduce

1. Take a hosted instance on a stack below Node-RED 4.0.2, for example 3.1.15.
2. Click Open Editor. The editor opens outside the immersive editor.
3. Look for the MCP pin toggle beside the Expert button. It is absent, as is the rest of the FlowFuse UI.
4. Change the instance to a Node-RED 4.x stack and restart. The control appears and pinning works.

### Environment

- FlowFuse version: FlowFuse Cloud, 11 September 2026
- Node-RED version: 3.1.15 (fails), 4.1.15 (works)
- Launcher version: 3.0.2
- Node.js version: 18.20.8
- Platform/OS: FlowFuse Cloud
- Browser: any

### Have you provided an initial effort estimate for this issue?

I can not provide an initial effort estimate

Start in frontend/src/pages/instance/components/EditorLink.vue at the Node-RED and Launcher version gate, then trace the affected instance/editor page path and the existing Expert update messaging. The change is complete when users on unsupported versions are told that MCP pinning needs a newer Node-RED and what action is required, without changing the existing feature-flag gating.

Written by the indexing model from the issue text.