test(e2e): golden-master Playwright net for the Platform refactor

Locks the five flows the refactor must not break (feed, channel, player,
settings, nav) so later sprints can prove they changed nothing. Adds a thin
data-testid layer and a dedicated seeded non-demo account; the suite runs on
the baked :8080 stack (authoritative) or :5173. No product-behaviour change.

frontend/src/components/VideoCard.tsx

@@ -448,6 +448,7 @@ function VideoCard({
   const channelButton = (
     <button
       ref={channelRef}
+      data-testid="video-card-channel"
       onClick={(e) => {
         e.stopPropagation();
         onOpenChannel?.(video.channel_id, video.channel_title ?? t("card.thisChannel"));
@@ -497,7 +498,7 @@ function VideoCard({
       const pct = resumePct(video);
       const fits = (needs: number) => width === 0 || width >= needs;
       return (
-        <div className={clsx(rowShell, "flex items-center gap-3 px-3 py-2", watched && "opacity-55")}>
+        <div data-testid="video-card" data-video-id={video.id} className={clsx(rowShell, "flex items-center gap-3 px-3 py-2", watched && "opacity-55")}>
           {/* Actions lead: this is where the eye already is (the title is the thing you read), so
               they're the shortest pointer trip from it. */}
           <div className="shrink-0 w-48">{actions}</div>
@@ -550,7 +551,7 @@ function VideoCard({
 
   if (spec.family === "row") {
     return (
-      <div className={clsx(rowShell, "flex gap-3 p-2", watched && "opacity-55")}>
+      <div data-testid="video-card" data-video-id={video.id} className={clsx(rowShell, "flex gap-3 p-2", watched && "opacity-55")}>
         <Thumb
           video={video}
           className={clsx("aspect-video shrink-0", spec.actionsBelow ? "w-40" : "w-44")}
@@ -580,6 +581,8 @@ function VideoCard({
   // by a pixel or two forever, which shows up as the whole page juddering.
   return (
     <div
+      data-testid="video-card"
+      data-video-id={video.id}
       className={clsx(
         "cv-card group glass-card glass-hover rounded-2xl transition-all duration-150 hover:-translate-y-1",
         "flex flex-col",