slider: Support `reverse` to fill the track from thumb to max end (#2541)

## Description

Adds a `reverse()` builder method to `Slider`. By default the track is
filled from the min end to the thumb; with `reverse`, the fill goes from
the thumb to the max end instead — useful when the slider represents a
remaining amount (e.g. time left in a media player).

```rust
Slider::new(&state).horizontal().reverse()
```

This only changes the visual fill; values, events and interactions are
unaffected. It applies to single-value sliders and is ignored for range
sliders (a range slider's fill is already bounded by both thumbs).

Also adds a "Reverse Slider" section to the slider story to showcase the
new option.

No breaking changes.

<img width="1642" height="1106" alt="image"
src="https://github.com/user-attachments/assets/c107d7e3-9856-4d27-a3c2-360d2d7ac0df"
/>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>