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/SettingsPanel.tsx

@@ -69,6 +69,7 @@ export default function SettingsPanel({
               return (
                 <button
                   key={tabItem.id}
+                  data-testid={`settings-tab-${tabItem.id}`}
                   onClick={() => setTab(tabItem.id)}
                   className={`flex items-center gap-2 px-3 py-2 rounded-lg text-sm text-left transition ${
                     active