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/Modal.tsx

@@ -56,7 +56,7 @@ export default function Modal({
 
   return createPortal(
     <div
-      className="fixed inset-0 z-overlay flex items-center justify-center p-4 sm:p-6 bg-black/70 backdrop-blur-xs"
+      className="fixed inset-0 z-overlay flex items-center justify-center p-4 sm:p-6 bg-black/70 overlay-backdrop"
       onMouseDown={(e) => {
         pressedOnBackdrop.current = e.target === e.currentTarget;
       }}

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">

frontend/src/components/PlayerModal.tsx

@@ -889,7 +889,7 @@ export default function PlayerModal({
     <div
       ref={dialogRef}
       data-testid="player-modal"
-      className="fixed inset-0 z-overlay flex items-center justify-center p-4 sm:p-6 bg-black/80 backdrop-blur-xs"
+      className="fixed inset-0 z-overlay flex items-center justify-center p-4 sm:p-6 bg-black/80 overlay-backdrop"
       onClick={onClose}
       role="dialog"
       aria-modal="true"

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"

frontend/src/index.css

@@ -479,6 +479,17 @@ html[data-perf="1"] body {
   background: var(--bg);
 }
 
+/* Full-viewport modal scrim blur, as a named recipe so perf mode can DROP it. This is the most
+   expensive blur in the app (viewport-sized); as a raw `backdrop-blur-xs` utility on each overlay
+   it survived data-perf while every .glass surface had its blur dropped. Overlays keep their own
+   bg-black/NN; this adds only the (perf-aware) 4px blur, matching backdrop-blur-xs. */
+.overlay-backdrop {
+  backdrop-filter: blur(4px);
+}
+html[data-perf="1"] .overlay-backdrop {
+  backdrop-filter: none;
+}
+
 /* Skip layout/paint for off-screen feed cards so long lists stay smooth while
    scrolling. contain-intrinsic-size reserves an approximate box so the scrollbar
    stays stable; `auto` lets the browser remember each card's real size after it