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/LanguageSwitcher.tsx

@@ -38,7 +38,7 @@ export default function LanguageSwitcher({
 
       {open && (
         <div
-          className={`glass absolute ${
+          className={`glass-menu absolute ${
             align === "right" ? "right-0" : "left-0"
           } mt-1 w-40 rounded-xl p-1.5 z-40 animate-[popIn_0.16s_ease]`}
         >