Improve treemap color scheme

There are two parts of this problem: single file analysis, and diff analysis.

Single file analysis:
- we could use a gradient (red for large code size nodes and gradually go down to yellow for smaller code size nodes)
- use meaningful colors to call out problems

Diff analysis:
- Red / green is confusing for users. We could use non red / green colors to show positive and negative changes in the diff, or we could flip the current scheme between red / green and their meanings
- maybe only use a red -> yellow gradient to show code size additions and some neutral color to show other negative changes in the diff

In general, we should try to use color and maybe some icons to call out potential code size problems

该 issue 未指定任何文件、测试或入口点。首先定位 treemap 的实现及其单文件视图和差异分析视图，然后检查当前颜色如何表示代码大小以及正向或负向变化。完成的标准是：形成一套达成共识的颜色和图标方案，能够清晰传达代码大小问题，而不依赖容易造成混淆的红/绿色含义。

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