Summary

• Enable using CSS linear-gradient for the Slider component's track background
• Implements feature request from [Feature] Slider linear-gradient support  #5940

Android Implementation

• Creates android.graphics.LinearGradient shader from CSS gradient color stops
• Applies gradient to ShapeDrawable wrapped in ClipDrawable for progress clipping
• Sets both progress layer and background layer (at 30% opacity) of the SeekBar

iOS Implementation

• Creates CAGradientLayer with colors and positions from CSS gradient
• Renders gradient layer to UIImage
• Creates stretchable track images using resizableImageWithCapInsetsResizingMode
• Sets minimum track image (full opacity) and maximum track image (30% opacity)

Usage

Via CSS

.gradient-slider {
    background: linear-gradient(to right, orangered, green, lightblue);
}

<Slider class="gradient-slider" minValue="0" maxValue="100" value="50" />

Programmatically

import { LinearGradient } from '@nativescript/core/ui/styling/linear-gradient';
import { Color } from '@nativescript/core';

const gradient = new LinearGradient();
gradient.angle = Math.PI / 2; // left to right
gradient.colorStops = [
    { color: new Color('orangered') },
    { color: new Color('green') },
    { color: new Color('lightblue') },
];

slider.style.backgroundImage = gradient;

Test plan

• Added unit tests for linear gradient backgrounds
• Added unit tests for linear gradients with explicit color stops
• Manual testing on Android device/emulator
• Manual testing on iOS device/simulator

Closes #5940