Create Instance button is not disabled after reaching max instance limit

### Current Behavior

Users can still click the "Create Instance" button even after reaching the maximum allowed number of instances. No tooltip is shown.

### Expected Behavior

The "Create Instance" button should be disabled when the maximum number of instances is reached. On hover, a tooltip should explain why it is disabled (e.g., "You’ve reached the maximum number of instances allowed for your plan").

### Steps To Reproduce

## Steps to reproduce
1. Create instances until reaching the maximum allowed by the plan
2. Observe the "Create Instance" button remains enabled
3. Hover over the button – no tooltip is displayed

### Environment

## Environment
- FlowFuse version: 2.20.0
- Node.js version: N/A
- npm version: N/A
- Platform/OS: N/A
- Browser: N/A

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

I have provided an initial effort estimate