SVG app icons don't load unless the URL ends in .svg

Since there's no `ImageSource` that can load both `.svg` and bitmap images, and instead there's a separate `SvgImageSource` and `BitmapImage`, we currently only attempt to load SVGs when the URL ends in `.svg`. We don't have much control over the actual downloading/caching operations of these sources.

One potential solution is to do a `HEAD` request first to get the `Content-Type` of the image, but it won't be easy to cache that result.

Another would be to download/cache images ourselves.

Relates to #92

首先跟踪基于 URL 的图像如何在 SvgImageSource 和 BitmapImage 之间进行选择，重点关注当前的 `.svg` 后缀检查。检查下载和缓存行为，然后比较 issue 中描述的 HEAD/Content-Type 和应用程序管理的缓存选项。当 URL 不以 `.svg` 结尾的 SVG 图标能够加载，且位图图像的加载不受影响时，即表示完成。

由索引模型根据 Issue 内容生成。