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/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