Sample layer visualization

[rozyczko]

New "Structure" tab on the Model page, providing a schematic visualization of the layer stack in the sample model. This view displays each layer as a colored box with interactive features and updates live as the model changes.

Frontend: Structure Tab and Visualization

• Added a "Structure" tab to the Model page, with a new StructureView.qml component that renders the schematic stack, including colored boxes per layer, tooltips, selection on click, a legend, and a total thickness caption. The view updates live on edits and after fits.
• Updated documentation to describe the new Structure view and its features.

Backend: Data Provision and Synchronization

• Implemented the flatten function in structure.py to generate the schematic layer stack, legend, and total thickness, with logic for collapsed repeating multilayers and color assignment.
• Exposed new properties (structure, structureLegend, structureTotalThickness) and change signals in the Python backend (sample.py), with caching and cache invalidation on relevant model changes.
• Connected model and fit-related signals to trigger updates of the structure view, ensuring it stays in sync with all relevant changes.

Mock Backend and QML Integration

• Added matching properties to the mock backend and integrated the new structure properties into the QML backend wrapper