feat(glass): glassify the side rails + back-to-top (Phase 1, feed chrome)

Convert the always-visible flat rails to the frosted glass system so they sit
consistently over the new background image (like the nav/header already do):
- Feed filter Sidebar + CollapsedFilterRail: bg-surface/40 -> glass
- Playlists left rail: bg-surface/40 -> glass
- BackToTop FAB: bg-card+border -> glass-card glass-hover
Small controls inside (filter pills/inputs) stay solid — a solid-on-glass
hierarchy, not everything translucent.

frontend/src/components/BackToTop.tsx

@@ -34,7 +34,7 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   return createPortal(
     <button
       onClick={toTop}
-      className={`fixed bottom-5 right-5 z-30 inline-flex items-center justify-center w-11 h-11 rounded-full bg-card border border-border text-fg shadow-lg hover:border-accent hover:text-accent transition-all duration-200 ${
+      className={`fixed bottom-5 right-5 z-30 inline-flex items-center justify-center w-11 h-11 rounded-full glass-card glass-hover text-fg shadow-lg hover:text-accent transition-all duration-200 ${
         show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
       }`}
     >