优化 ScrollBar 样式

[CiiLu]

感觉有点过于圆+粗了

[Calboot]

恭喜你，你成功的 reverse 了 #5716

之前调粗是为了防止被吞圆角，比如

这里你调细就会出问题

不幸的是，我没有在 m3 中找到任何滚动条的描述

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当然，也有一种解决办法是让它不要顶到头上

另外如果你执意要这么改的话建议改成 4，因为原来就是 4（

[Glavo]

/gemini review

[Glavo]

我感觉改之后又有点太细了（可能是习惯了新的样式），我感觉还是粗一点容易操控。

[gemini-code-assist]

Code Review

This pull request adjusts the scroll bar's appearance by reducing its thickness from 8 to 4.5 and introducing a layout offset. The changes affect both the Java skin implementation and the CSS styling. Feedback suggests improving code maintainability by replacing hardcoded values with constants or variables and refactoring redundant binding logic for the track and thumb components to reduce duplication.

[gemini-code-assist]

使用前面定义的 thickness、isHorizontal 和 trackLength 变量来简化滑块（thumb）的配置，避免重复的逻辑判断和绑定计算。

                if (isHorizontal) {
                    thumb.relocate(0, -thickness);
                    thumb.setHeight(thickness);

                    thumb.widthProperty().bind(Bindings.max(20, scrollBar.visibleAmountProperty().divide(range).multiply(trackLength)));
                    thumb.xProperty().bind(
                            Bindings.add(offset,
                                    Bindings.subtract(trackLength, thumb.widthProperty()).multiply(position))
                    );
                } else {
                    thumb.relocate(-thickness, 0);
                    thumb.setWidth(thickness);

                    thumb.heightProperty().bind(Bindings.max(20, scrollBar.visibleAmountProperty().divide(range).multiply(trackLength)));
                    thumb.yProperty().bind(
                            Bindings.add(offset,
                                    Bindings.subtract(trackLength, thumb.heightProperty()).multiply(position))
                    );
                }