Refactor SLD and molecular weight dependency in app structure

[rozyczko]

This pull request introduces support for "density materials" (materials where SLD/ISLD are derived from density and formula, but can be decoupled to fit SLD/ISLD directly).

Backend logic for density materials:

• Added helper functions _is_density_material and _sld_is_writable to distinguish density materials and control SLD/ISLD writability.
• Extended the Material class with methods to set sld_coupled, formula, and density for density materials, with appropriate guards and error handling.

Parameter and serialization handling:

• Updated parameter list construction to mark density-material knobs as "inactive" (greyed out, not fitted) when decoupled, and added notes for their state.
• Enhanced the material serialization to include kind, formula, density, and coupling state for each material.

API and QML interface updates:

• Added new slots and QML wrapper functions to set sld_coupled, formula, and density for materials from the UI, and to emit appropriate signals for UI updates and constraint invalidation.

UI improvements:

• Improved error formatting in the analysis sidebar to display 'n/a' for missing error bars, distinguishing this from numeric zero.