feat(slider): add support for linear gradients for min/max track

Add support for applying LinearGradient instances to a Slider's min and max tracks
on Android and iOS.

• Export and register new properties minTrackGradient and maxTrackGradient on SliderBase.
• Implement Android shader-backed LayerDrawable that substitutes the progress layer with a shader
  drawable while preserving original layer IDs.
• Implement iOS rendering by rasterizing CAGradientLayer into a resizable track image with
  proper scale, corner radius and angle handling.
• Add unit tests and automated UI tests to validate property exports, native drawable/image
  retrieval and gradient updates.
• Add small accessibility/events cleanup and shared index refactor.

Notes:

• Restores original drawables/images when gradient is set to null.
• Gradient offsets are handled (percent → 0..1) and color stops are evenly distributed when
  offsets are absent.

Breaking changes: none

Closes: #5940