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.