fix: remove margin-left for single a tag in SocialLinks component

[huyixi]

This commit fixes an issue where a single tag in the SocialLinks component would have an unwanted margin-left, causing misalignment. The issue is resolved by using the :only-child pseudo-class to remove the margin-left when there is
only one tag.

Updated index.module.scss to include the necessary CSS selector.

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[Timeless0911]

before

after

You can see the display. Your PR makes the left and right spacing not look well when there is only one social link.

The margins to the theme switch button are too small.

[huyixi]

before

after

You can see the display. Your PR makes the left and right spacing not look well when there is only one social link.

The margins to the theme switch button are too small.

I forgot to consider the situation on large screens. The new PR has fixed this issue

before:

after: