gpui: Make image auto sizing work with Rems too (#40089)

Closes #39981

Here both images of the left should be identical to the right, since
180px is the same as 11.25rem.

Before:

<img width="1457" height="847" alt="image"
src="https://github.com/user-attachments/assets/59f571d1-8d66-4f41-b9b0-e9826110cf0c"
/>

After:

<img width="1457" height="626" alt="image"
src="https://github.com/user-attachments/assets/a0c629a9-5916-453a-85a2-b3053ab2e613"
/>

Release Notes:

- N/A
785cb41565Smit Barmase committed on 10/13/2025, 11:22:33 AM· committed by GitHubparentce20e71
1 file changedLine totals unavailable