Wire social-share hook + make Follow button provider-aware

The social-share buttons function was hooked to
projects_after_download_button, but the template never fired that action —
the buttons never rendered. Added a do_action() call after the View Repo
button.

The Follow button hardcoded a GitHub URL and used the GitHub-specific
github-button class (which only acts on github.com). Replaced with a
plain styled link pointing at the owner's actual URL (works for any
provider). Removed the assets/js/buttons.js enqueue since nothing uses
the github-button class anymore.

Renamed the project-github-button CSS class on the View Repo link to
project-repo-button since it's provider-neutral now.

templates/single-projects.php

@@ -69,12 +69,13 @@ if ( $last_updated ) {
                     <?php esc_html_e( 'Download Now', 'projects-wp' ); ?>
                 </a>
                 <?php if ( $repo_url ) : ?>
-                    <a href="<?php echo esc_url( $browse_url ); ?>" class="button project-github-button" target="_blank" rel="noopener noreferrer">
+                    <a href="<?php echo esc_url( $browse_url ); ?>" class="button project-repo-button" target="_blank" rel="noopener noreferrer">
                         <?php esc_html_e( 'View Repo', 'projects-wp' ); ?>
                     </a>
                 <?php else : ?>
                     <p><?php esc_html_e( 'Repository:', 'projects-wp' ); ?> <?php esc_html_e( 'No repository linked.', 'projects-wp' ); ?></p>
                 <?php endif; ?>
+                <?php do_action( 'projects_after_download_button', $project_id ); ?>
             </div>
 
             <div class="project-meta">
@@ -151,7 +152,7 @@ if ( $last_updated ) {
                         </strong>
                     </span>
                     <span>
-                        <a class="github-button" href="https://github.com/<?php esc_html_e( $owner_name ); ?>" data-show-count="true" data-color-scheme="no-preference: light; light: light; dark: dark;" data-size="large" aria-label="Follow @<?php esc_html_e( $owner_name ); ?> on GitHub"><?php esc_attr_e( 'Follow', 'projects-wp' ) ?></a>
+                        <a class="button project-follow-button" href="<?php echo esc_url( $owner_url ); ?>" target="_blank" rel="noopener noreferrer"><?php esc_html_e( 'Follow', 'projects-wp' ); ?></a>
                     </span>
                 </div>
             <?php endif; ?>