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/components/VideoCard.tsx

@@ -1,3 +1,4 @@
+import { memo } from "react";
 import { Bookmark, Check, CheckCheck, Eye, EyeOff, ListFilter } from "lucide-react";
 import Avatar from "./Avatar";
 import clsx from "clsx";
@@ -136,7 +137,7 @@ function Thumb({
   );
 }
 
-export default function VideoCard({
+function VideoCard({
   video,
   view,
   onState,
@@ -172,7 +173,7 @@ export default function VideoCard({
     return (
       <div
         className={clsx(
-          "group flex gap-3 p-2 rounded-xl hover:bg-card hover:shadow-lg transition",
+          "cv-row group flex gap-3 p-2 rounded-xl hover:bg-card hover:shadow-lg transition",
           watched && "opacity-55"
         )}
       >
@@ -198,7 +199,7 @@ export default function VideoCard({
   return (
     <div
       className={clsx(
-        "group glass-card glass-hover rounded-2xl p-2.5 transition-all duration-150 hover:-translate-y-1",
+        "cv-card group glass-card glass-hover rounded-2xl p-2.5 transition-all duration-150 hover:-translate-y-1",
         watched && "opacity-55"
       )}
     >
@@ -227,3 +228,8 @@ export default function VideoCard({
     </div>
   );
 }
+
+// Memoized so appending a feed page only renders the new cards, not every existing
+// one (the callbacks from Feed are stable; non-overridden video objects keep their
+// identity across renders).
+export default memo(VideoCard);