gpui: Maintain img aspect ratio when max_width is set (#25632)

Release Notes:

- Fixed Markdown preview to display image with max width 100%.

## Before

<img width="1202" alt="image"
src="https://github.com/user-attachments/assets/359628df-8746-456f-a768-b3428923c937"
/>
<img width="750" alt="SCR-20250226-napv"
src="https://github.com/user-attachments/assets/f6154516-470e-41b2-84f5-ef0612c447ad"
/>


## After

<img width="1149" alt="image"
src="https://github.com/user-attachments/assets/2279347d-9c69-4a47-bb62-ccc8e55a98f6"
/>
<img width="520" alt="SCR-20250226-ngyz"
src="https://github.com/user-attachments/assets/03af5f14-1935-472e-822f-4c7f62630780"
/>

crates/gpui/src/elements/img.rs

@@ -301,6 +301,8 @@ impl Element for Img {
                             }
 
                             let image_size = data.size(frame_index);
+                            style.aspect_ratio =
+                                Some(image_size.width.0 as f32 / image_size.height.0 as f32);
 
                             if let Length::Auto = style.size.width {
                                 style.size.width = match style.size.height {