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/PlayerModal.tsx

@@ -591,6 +591,7 @@ export default function PlayerModal({
   return createPortal(
     <div
       ref={dialogRef}
+      data-testid="player-modal"
       className="fixed inset-0 z-50 flex items-center justify-center p-4 sm:p-6 bg-black/80 backdrop-blur-sm"
       onClick={onClose}
       role="dialog"
@@ -801,6 +802,7 @@ export default function PlayerModal({
                 document.body
               )}
             <button
+              data-testid="player-close"
               onClick={onClose}
               title={t("player.closeEsc")}
               className="shrink-0 inline-flex items-center gap-1.5 text-sm px-3 py-1.5 rounded-lg text-muted hover:text-fg hover:bg-surface transition"