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/Sidebar.tsx

@@ -462,7 +462,7 @@ export default function Sidebar({
   }
 
   return (
-    <aside className="hidden md:block w-64 shrink-0 border-r border-border bg-surface/40 overflow-y-auto p-4 space-y-3">
+    <aside className="hidden md:block w-64 shrink-0 glass overflow-y-auto p-4 space-y-3">
       {/* Scope: your own subscriptions (Mine) vs the shared Library — moved out of the top bar. */}
       <div
         className="flex items-center rounded-full border border-border bg-card p-0.5 text-xs"