fix: clear BackToTop float to prevent nav layout shift

[mathieumaf]

When SHARE_LINKS is empty, the BackToTopButton's md:float-end was not cleared before the prev/next navigation grid, pushing buttons out of alignment. Adding clear-end on the separating <hr> fixes this.

Fixes #614

Description

When SHARE_LINKS is empty in src/constants.ts, the ShareLinks component renders nothing. This means the BackToTopButton's md:float-end is not cleared before the prev/next navigation grid, causing the "Next Post" / "Previous Post" buttons to shift left instead of staying properly aligned.

The fix adds clear-end on the <hr> that separates the share links section from the navigation, ensuring the float is always cleared regardless of whether share links are present.

Types of changes

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Documentation Update (if none of the other choices apply)
• Others (any other types not listed above)

Checklist

• I have read the Contributing Guide
• I have added the necessary documentation (if appropriate)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)

Further comments

Single-line change in src/layouts/PostDetails.astro (line 129):

- <hr class="my-6 border-dashed" />
+ <hr class="my-6 clear-end border-dashed" />

Tested both with empty and populated SHARE_LINKS — no regression.

Related Issue

Closes: #614