Add SliderContent RenderFragment parameter to VerticalSliderPanel

[Copilot]

This PR adds the missing SliderContent RenderFragment parameter to the VerticalSliderPanel component to achieve parity with the HorizontalSliderPanel.

Problem

The HorizontalSliderPanel component included a SliderContent parameter that allows developers to customize the content displayed within the slider bar, but this functionality was missing from the VerticalSliderPanel.

Solution

Added the SliderContent RenderFragment parameter to VerticalSliderPanel:

1. Added parameter declaration in VerticalSliderPanel.razor.cs:

   [Parameter]
   public RenderFragment SliderContent { get; set; }

2. Added content rendering in VerticalSliderPanel.razor:

   <div id="@SliderId" class="@DefaultSliderClass @SliderClassString" style="...">
       @SliderContent
   </div>

Testing

• Updated the CustomClasses demo page to showcase the new functionality with a custom gradient slider
• Verified both components build successfully
• Confirmed existing functionality remains unaffected
• Visual testing shows the SliderContent renders correctly

Parameter Parity Analysis

Conducted a comprehensive analysis of both components and documented the findings in PARAMETER_PARITY_ANALYSIS.md. The components now have appropriate parity, with remaining differences being intentional design choices based on orientation (horizontal vs vertical layouts).

Example Usage

<VerticalSliderPanel LeftPanelStartingWidth="300">
    <LeftChildContent>Left panel content</LeftChildContent>
    
    <SliderContent>
        <div style="background: #007acc; color: white; text-align: center;">
            ⇔ DRAG ⇔
        </div>
    </SliderContent>
    
    <RightChildContent>Right panel content</RightChildContent>
</VerticalSliderPanel>

Fixes #27.

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