perf(feed): smoother scrolling for long feeds

Three low-risk wins for large filtered feeds:
- content-visibility:auto on cards (.cv-card/.cv-row) so the browser skips
  layout/paint for off-screen cards; contain-intrinsic-size keeps the scrollbar
  stable and is remembered per card after first render.
- memo(VideoCard) + stable onState/onChannelFilter callbacks (onState reads the
  loaded list via a ref) so appending a page only renders the ~60 new cards
  instead of reconciling every card already on screen.
- Prefetch the next page earlier (sentinel rootMargin 800px → 1500px) so the
  'Loading more…' flash is far less likely during fast scrolling.

frontend/src/index.css

@@ -72,6 +72,19 @@ html[data-perf="1"] body {
   background: var(--bg);
 }
 
+/* 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
+   has rendered once. */
+.cv-card {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 320px;
+}
+.cv-row {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 96px;
+}
+
 /* ===== Motion ===== */
 @keyframes fadeIn {
   from { opacity: 0; }