Fix preload image link behavior for covers array and single cover

[szNightFury]

When preloading cover images in the header, there are two issues with the current implementation:

1. For multiple covers (array), the href attribute contains the entire array stringified rather than individual image URLs:

<!-- Current incorrect output -->
<link rel="preload" href="[&quot;https://example.com/1.jpg&quot;,&quot;https://example.com/2.jpg&quot;]" as="image">

2. For a single cover (string), the string is incorrectly iterated character by character, resulting in multiple preload links for each character:

<!-- Current incorrect output -->
<link rel="preload" href="/" as="image">
<link rel="preload" href="c" as="image">
<link rel="preload" href="o" as="image">
<link rel="preload" href="v" as="image">

Expected Behavior

• For multiple covers: Each image URL should have its own preload link
• For single cover: Should only generate one preload link for the entire URL

Proposed Solution

Update the template logic to handle both array and string cases:

if enablePreload && enableCover
    if enableFixedCover
        link(rel="preload" href=theme.homeConfig.fixedCover as="image" fetchpriority="high")
    else if Array.isArray(covers)
        each image in covers
            link(rel="preload" href!=image as="image" fetchpriority="high")
    else
        link(rel="preload" href!=covers as="image" fetchpriority="high")