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/Sidebar.tsx

@@ -37,6 +37,8 @@ function TagChip({
   const { t } = useTranslation();
   return (
     <button
+      data-testid="filter-tag-chip"
+      data-tag={tag.name}
       onClick={onClick}
       title={t("sidebar.channelCount", { count })}
       className={`inline-flex items-center gap-1.5 text-xs px-2.5 py-1 rounded-full border shadow-sm hover:shadow active:translate-y-px transition ${