fix(frontend): prevent back-to-top button overlapping footer (#4966)

[Shekhar0109]

Fixes

Fixes #4966 by @zackkrida

Description

This PR fixes a regression where the “Back to top” button overlapped the footer on Openverse pages.
Because the button used position: fixed and was placed outside the scroll container, it covered the footer content in both light and dark mode.

What was changed

• Moved <VScrollButton /> inside the main-page container in both:
  • SearchLayout.vue
  • ContentLayout.vue
• Changed the button’s CSS in VScrollButton.vue from position: fixed to position: sticky
• Ensures the button scrolls naturally with page content and no longer overlaps footer controls

Testing Instructions

1. Navigate to any search results page (or content page).
2. Scroll down until the “Back to top” button appears.
3. Continue scrolling to the footer.
4. Verify that:
   • The button no longer overlaps or hides footer controls.
   • The button remains visible until reaching the footer area.
   • Clicking the button scrolls smoothly to the top.

Checklist

• My pull request has a descriptive title.
• My pull request targets the main branch.
• My commit messages follow best practices.
• My code follows the repository's style.
• I updated relevant files in components and layouts.
• I attempted running the project locally (not possible on my environment due to missing Docker).
• The PR fixes the issue fully.

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