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.


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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

Start by tracing the Windows desktop app's SVG icon loading through WinUI 3's SvgImageSource, using /icon/cursor.svg and /icon/code.svg as reproduction cases. Check how embedded elements and gradients are handled, then verify that affected icons render correctly without silent failure.

Written by the indexing model from the issue text.