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

Start by tracing how URL-based images select between SvgImageSource and BitmapImage, focusing on the current `.svg` suffix check. Review the downloading and caching behavior, then compare the HEAD/Content-Type and application-managed caching options described in the issue. Done means SVG icons load when their URLs do not end in `.svg`, without breaking bitmap image loading.

索引モデルが issue の本文から書いたものです。