Add the variable widget

A Add the variables widget to the UI, placeholders for the code it will run (which will be done by bluesky plans) and connect it to the main UI

<img width="299" height="269" alt="Image" src="https://github.com/user-attachments/assets/cded8345-3fd6-4bc2-b62f-87950c03b453" />


## Acceptance Criteria
- The variable widget exists, is connected but for now only mocks the requested behaviour
