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/Playlists.tsx

@@ -524,7 +524,7 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
 
   return (
     <div className="flex h-full min-h-0">
-      <aside className="w-64 shrink-0 border-r border-border bg-surface/40 overflow-y-auto p-3">
+      <aside className="w-64 shrink-0 glass overflow-y-auto p-3">
         <div className="flex items-center justify-between mb-2">
           <span className="text-sm font-semibold">{t("playlists.title")}</span>
           <button