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/NotificationsPanel.tsx

@@ -255,7 +255,7 @@ function NotificationRow({
   }
   return (
     <div
-      className={`glass rounded-xl p-3.5 flex items-start gap-3 transition ${
+      className={`glass-card rounded-xl p-3.5 flex items-start gap-3 transition ${
         n.read ? "opacity-70" : ""
       }`}
     >
@@ -340,7 +340,7 @@ function ClientActivityRow({
   const access = n.meta?.kind === "access-requests" ? n.meta : null;
   return (
     <div
-      className={`glass rounded-xl p-3.5 flex items-start gap-3 ${needsAction ? "ring-1 ring-accent/30" : ""}`}
+      className={`glass-card rounded-xl p-3.5 flex items-start gap-3 ${needsAction ? "ring-1 ring-accent/30" : ""}`}
     >
       <Icon className={`w-4 h-4 shrink-0 mt-0.5 ${color}`} />
       <div className="min-w-0 flex-1">