Apply aspectRatio to AsyncImage placeholder

[dfabulich]

Fixes #320 reproduced in skiptools/skipapp-showcase#59

The layout still doesn't perfectly match iOS, and I guess that's a separate bug, but this PR is definitely fixing something, for now.

Specifically, I'm unclear why the layout is so different between the "No URL" and the "Failed URL" case. I think the answer is that coil3.compose.SubcomposeAsyncImage is doing some layout trickery of its own.

iOS

Android on skip-ui 1.47.0

After applying #321

Skip Pull Request Checklist:

• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
• OPTIONAL: I have tested my change on an Android emulator or device
• REQUIRED: I have checked whether this change requires a corresponding update in the Skip Fuse UI repository (link related PR if applicable)
• OPTIONAL: I have added an example of any UI changes in the Showcase sample app