Header: Follow on Spotify button gets the Spotify icon, solid gold style, hover/focus states and a bigger mobile tap target; v2.23.2

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/App.tsx

@@ -495,6 +495,7 @@ function SiteHeader({ content }: { content: SiteContent }) {
             rel="noreferrer"
             className="header-cta"
           >
+            <SpotifyIcon />
             {content.headerFollowLabel || 'Follow on Spotify'}
           </a>
         </nav>