feat(CV): current density axis toggle

[Nicolass67]

This PR implements backend support for the Current / Current Density mode in the CV layout.

It introduces:

• Parsing and persistence of new CV parameters:
  • areaValue
  • areaUnit
  • useCurrentDensity
• Writing these parameters into the JCAMP metadata:
  • ##$CSWEAREAVALUE
  • ##$CSWEAREAUNIT
  • ##$CSCURRENTMODE
• Automatic Y-axis label override when current density mode is enabled.
• Dynamic Y-axis scaling and formatting when current density is active, including area-based normalization and scientific notation handling.

When useCurrentDensity is enabled, the backend converts the current to current density based on the provided electrode area and adapts both metadata and rendering accordingly.

This ensures consistent data export, correct labeling, and proper visualization behavior for CV current density mode.