Add spin arrows

[rozyczko]

Magnetic Moment Direction Visualization and Data Model:

• The backend (layers.py, structure.py) now computes and exposes the physical moment direction (phi), a flag for editability, and a threshold for negligible moments. These are included in the data structures sent to the frontend. The moment direction is omitted for non-magnetic and gradient layers, and a flag (has_moment) distinguishes negligible moments.

• The structure view (StructureView.qml) displays an arrow in each magnetic layer's box, with size constraints and visibility based on the moment's magnitude. A "H →" guide field reference is shown wherever arrows appear, clarifying the angle's reference direction.

• New QML components provide reusable, consistent visuals:

  • MagnetizationArrow.qml draws the moment arrow or a hollow dot for negligible moments.
  • GuideFieldLegend.qml displays the "H →" reference with tooltip.
  • MagnetizationAngleSlider.qml offers a 0-360° slider for the selected layer's angle, showing both θM and the derived φ, and disables editing when constrained.

UI/UX Consistency and Live Updates:

• Edits to magnetic parameters now trigger a full refresh of the structure view, ensuring arrows and tooltips always reflect the current state without stale angles.