fix(theme-default): ensure proper sizing of SVG hero images

[GabrielxDuO]

Description

When using vector graphics (SVGs) without specified dimensions as hero images, they can render at unexpectedly small sizes in certain viewport widths. This creates inconsistent presentation compared to bitmap images.

Before/After Comparison

	WebP	SVG
Before
After

[GabrielxDuO]

Yes, I also tried using the following approach:

width: 100%;
height: 100%;
object-fit: contain;

However, thinking back to when I tested it with vitepress-logo-large.webp, the inconsistency might have been caused by an outer glow or shadow (as opposed to the glow in vitepress-logo-large.svg, which is achieved using filter), making the .webp version appear slightly larger and therefore seem to scale down more noticeably.

That said, considering the broader issue that the hero image may not always be square, this solution (object-fit: contain) is indeed better suited. Thank you for making these adjustments!

[brc-dd]

Yeah, and the webp was also of older version I think.

(left one is svg, right is webp)

Also the webp had padding inside the image itself, and in svg it's just padding: 18px from css. So some difference might be because of that 👀