Add intentional color coordination

Right now colors are determined by the order you add them to the schedule, and eventually become entirely black. It would be really cool to have control over what colors each item in the schedule is, including non-class scheduled items.

该 issue 未指出任何文件、测试或入口点。首先定位日程项渲染和颜色分配逻辑，然后定义用户如何为课程项和非课程项选择颜色，并验证这些选择仍然彼此区分，而不是变成黑色。

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