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/NavSidebar.tsx

@@ -236,6 +236,7 @@ export default function NavSidebar({
     return (
       <button
         key={p}
+        data-testid={`nav-${p}`}
         onClick={() => setPage(p)}
         title={slim ? label : undefined}
         aria-current={active ? "page" : undefined}
@@ -350,6 +351,7 @@ export default function NavSidebar({
           </button>
         </div>
         <button
+          data-testid="nav-settings"
           onClick={() => setPage("settings")}
           title={slim ? t("header.account.settings") : undefined}
           aria-current={page === "settings" ? "page" : undefined}