fix(ui): opaque .glass-menu for floating popovers (readability)

Floating menus hover over undimmed content (no backdrop scrim like dialogs), so the
frosted .glass (78%) let the content bleed through and hurt readability. Add a near-opaque
.glass-menu (surface 92%, keeps blur) and use it for the account, language, notification
and add-to-playlist popovers. Dialogs/chrome keep the frostier .glass (they sit over a dark
scrim / the ambient bg). perf-mode disables its blur too.

frontend/src/components/AddToPlaylist.tsx

@@ -139,7 +139,7 @@ export default function AddToPlaylist({
           <div
             ref={panelRef}
             style={{ position: "fixed", top: coords.top, left: coords.left, width: 240 }}
-            className="z-50 glass rounded-xl p-2 shadow-2xl"
+            className="z-50 glass-menu rounded-xl p-2 shadow-2xl"
             onClick={(e) => e.stopPropagation()}
           >
             <div className="text-xs text-muted px-1.5 pb-1.5">