Avatar images no longer shift shape while loading

Fixed the visual glitch where avatars collapse from a tall pill to a square once the image finishes loading.

Bug Fixes

Avatar images were collapsing from a tall pill shape to a proper square as soon as the image loaded. This happened because the base-ui Avatar component mounts both the image and fallback text simultaneously during the loading state (unlike Radix, which renders only one). With flexbox layout, the two children were splitting the available width 50/50, creating the stretched appearance. Once the image loaded and the fallback disappeared, the layout would jump to fill the full square — a jarring visual flicker.

The fix switches to absolute positioning, so the image and fallback overlap within the same square frame rather than competing for space. The loading state now shows the fallback without distortion, and the transition when the image arrives is seamless.