refactor(glass): R10 S3a — glass-card notification rows + perf-aware .overlay-backdrop

- NotificationsPanel per-row .glass -> glass-card (the rows are cards, not panel surface).
- New .overlay-backdrop recipe (4px blur = backdrop-blur-xs) with a data-perf override that
  DROPS the blur. The viewport-sized modal scrim blur (Modal/PlayerModal/Welcome) was a raw
  backdrop-blur-xs utility that survived perf mode while every .glass had its blur dropped —
  the most expensive blur in the app. Overlays keep their bg-black/NN.
- gotcha: write ONLY the standard backdrop-filter; a manual -webkit- line makes lightningcss
  drop the standard prop (webkit-only output -> no blur in Firefox).

frontend/src/components/Welcome.tsx

@@ -258,7 +258,7 @@ function Lightbox({ src, label, onClose }: { src: string; label: string; onClose
   }, [onClose]);
   return (
     <div
-      className="fixed inset-0 z-popover grid place-items-center bg-black/80 backdrop-blur-xs p-4 sm:p-8"
+      className="fixed inset-0 z-popover grid place-items-center bg-black/80 overlay-backdrop p-4 sm:p-8"
       onClick={onClose}
       role="dialog"
       aria-modal="true"