Some SVG features aren't supported for SVG app icons

### Embedded bitmaps

Some SVG files are actually PNGs masquerading as SVGs.

Take for example `/icon/cursor.svg` in Coder:
```svg

```

WinUI 3's `SvgImageSource` doesn't seem to be able to render `` tags like these in SVGs, so `cursor.svg` shows up as blank.

There's no way to detect that it failed to render as it fails silently.

### VS Code Icon gradient

The VS Code icon `/icon/code.svg` has a weird white gradient over it in the app that doesn't appear when viewing the SVG in a web browser.


-----

We could fix all of the SVGs in Coder and just declare that the desktop app for windows can't render icons like these somewhere in our documentation or something, or we could parse the SVG to look for these tags and try to render as a `BitmapImage` instead.

Relates to #91

首先，通过 WinUI 3's SvgImageSource 跟踪 Windows 桌面应用的 SVG 图标加载过程，并使用 /icon/cursor.svg 和 /icon/code.svg 作为复现案例。检查嵌入的 元素和渐变的处理方式，然后验证受影响的图标能够正确渲染，且不会静默失败。

由索引模型根据 Issue 内容生成。