ui: Render fallback icon for avatars that failed to load (#43183)

Before we were simply not rendering anything which could lead to some
very surprising situations when joining channels ...

Now it will look like so
<img width="147" height="50" alt="image"
src="https://github.com/user-attachments/assets/13069de8-3dc0-45e1-b562-3fe81507dd87"
/>

Release Notes:

- Improved rendering of avatars that failed to load by rendering a
fallback icon
e6e5ccbf10Lukas Wirth committed on 11/20/2025, 7:30:34 PM· committed by GitHubparentd6d967f
1 file changedLine totals unavailable