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/ChannelPage.tsx

@@ -244,6 +244,7 @@ export default function ChannelPage({
           />
         )}
         <button
+          data-testid="channel-back"
           onClick={onBack}
           className="absolute top-5 left-7 sm:left-9 z-10 inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-black/45 text-white hover:bg-black/65 backdrop-blur-sm"
         >
@@ -263,7 +264,7 @@ export default function ChannelPage({
           />
           <div className="flex-1 min-w-0 pb-1">
             <div className="flex items-center gap-2 flex-wrap">
-              <h1 className="text-xl font-semibold truncate">{name}</h1>
+              <h1 data-testid="channel-title" className="text-xl font-semibold truncate">{name}</h1>
               {ch?.blocked ? (
                 <span className="text-xs px-2 py-0.5 rounded-full bg-danger/15 text-danger">
                   {t("channel.blockedBadge")}