Convert Gallery SCSS to CSS

[body-clock]

Closes #193

We're waiting until after we cut a v4 release and move main target v5 to merge this.

[Copilot]

Pull Request Overview

This PR converts Gallery SCSS files to plain CSS by replacing SCSS features (such as @extend and SCSS variables) with equivalent CSS classes and CSS variables. It also adjusts HTML markup to utilize Bootstrap spacing utilities.

• Converted SCSS styles to CSS by removing @extend statements and SCSS variables in favor of CSS variables and explicit style rules.
• Updated the component markup to include the "mt-2" class for margin adjustments.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
app/components/blacklight/gallery/document_component.html.erb	Added "mt-2" class to the caption container to adjust spacing
app/assets/stylesheets/blacklight_gallery/_gallery.css	Replaced SCSS constructs (@extend, SCSS variables) with CSS equivalents

Comments suppressed due to low confidence (1)

app/assets/stylesheets/blacklight_gallery/_gallery.css:41

• The new usage of CSS variables (var(--bs-border-width) and var(--bs-border-color)) assumes that these variables are defined and available globally. Please verify that these variables are properly configured in the project to ensure consistent styling.

.documents-gallery,