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/NavSidebar.tsx

@@ -143,7 +143,7 @@ export default function NavSidebar({
           {acctOpen && (
             <>
               <div className="fixed inset-0 z-20" onClick={() => setAcctOpen(false)} />
-              <div className="glass absolute left-full bottom-0 ml-2 w-60 rounded-xl p-3 z-30 animate-[popIn_0.16s_ease]">
+              <div className="glass-menu absolute left-full bottom-0 ml-2 w-60 rounded-xl p-3 z-30 animate-[popIn_0.16s_ease]">
                 <div className="flex items-center gap-3 pb-3 border-b border-border">
                   <AvatarImg
                     src={me.avatar_url}