Improve web app visualization

- [ ] Plumb along and display zones for origins/destinations too?
- [ ] For zone-based origins/destinations, also visualize the `od.csv` outputs
- [ ] Scale O/D circles based on count, and include histograms for them too?
- [ ] Try rank/ordering for line width (1st most important edge, 2nd...)
- [ ] Auto-set the max for line width value, by plumbing along the max count in the output metadata

这个 issue 是为了增强 Web 应用对起讫点数据的可视化。查看处理 od.csv 输出的代码以及当前的可视化逻辑。首先找到处理区域和计数的位置，然后修改 frontend，使圆圈大小和线宽相应缩放。检查可视化组件的现有测试，以了解预期行为。

由索引模型根据 Issue 内容生成。