Setting a default Stack for Instance creation in FlowFuse

### Description

## User Story

As an Admin of FlowFuse,

I want the ability to select a default Node-RED stack for instance creation,

So that the instance creation process is streamlined and efficient

## Scope

- Fixed Stack During Creation: When creating an instance, the Node-RED stack (e.g., version 3.1) should be fixed and not changeable / visible during setup.

- Option to Change Stack Post-Creation: Users should have the option to select a different Node-RED stack via Settings after the instance is created, to accommodate specific needs or updates.

- Simplified Process: The default stack selection should simplify the instance creation process, reducing steps and decisions for users, leading to a more efficient workflow.

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

I have provided an initial effort estimate