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/Feed.tsx

@@ -538,6 +538,7 @@ export default function Feed({
         {SHOW_IDS.map((id) => (
           <button
             key={id}
+            data-testid={`feed-show-${id}`}
             onClick={() => setFilters({ ...filters, show: id })}
             aria-pressed={filters.show === id}
             className={`text-xs px-3 py-1.5 rounded-full border transition ${
@@ -595,7 +596,7 @@ export default function Feed({
           </>
         )}
         <span className="mx-1 h-5 w-px bg-border" aria-hidden="true" />
-        <span className="text-xs text-muted whitespace-nowrap">
+        <span data-testid="feed-result-count" className="text-xs text-muted whitespace-nowrap">
           {countQuery.data
             ? t("feed.videoCount", {
                 count: countQuery.data.count,