fix: scroll-to-top visible class + reading progress dark mode

- Wire visible state to className in ScrollToTop component
- Replace hard-coded rgba with color-mix() for theme-aware background
- Dark mode reading progress uses 15% ink color mix

components/ui/ScrollToTop.tsx

@@ -19,7 +19,7 @@ export function ScrollToTop() {
       onClick={() => window.scrollTo({ top: 0, behavior: "smooth" })}
       whileHover={{ scale: 1.1 }}
       whileTap={{ scale: 0.95 }}
-      className="scroll-to-top"
+      className={`scroll-to-top ${visible ? 'visible' : ''}`}
       aria-label="Scroll to top"
     >
       <svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><polyline points="18 15 12 9 6 15"/></svg>