Add strokeColor property to forecastDataPoints options

[Grafikart]

New Pull Request

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Summary

• Adds a new strokeColor property to forecastDataPoints that allows customizing the stroke color of forecast data points separately from the main series color
• Works for line, area, and bar chart types

Motivation

Currently, forecast data points can be styled with custom dashArray, strokeWidth, and fillOpacity, but the stroke color always inherits from the series. This change allows users to visually distinguish forecast data with a different color. (This is a requirement for one of my project ^^)

Usage

 forecastDataPoints: {
   count: 7,
   dashArray: 4,
   fillOpacity: 0.5,
   strokeColor: '#ff0000'  // New property
}

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• My branch is up to date with any changes from the main branch