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/e2e/player.spec.ts

@@ -0,0 +1,28 @@
+import { test, expect } from "@playwright/test";
+import { openApp, openFirstVideo } from "./helpers";
+
+// Locks the in-app player's open/close contract. The player is a portaled overlay whose
+// inconsistent mounting caused an E3 regression — exactly the kind of thing the refactor's
+// overlay-root convention must not break. We assert the modal shell, not YouTube playback.
+test.describe("player modal", () => {
+  test.beforeEach(async ({ page }) => {
+    // Hermetic: don't actually load the YouTube iframe/player.
+    await page.route(/youtube\.com/, (route) => route.abort());
+  });
+
+  test("opens from a card and closes via the close button", async ({ page }) => {
+    await openApp(page);
+    await openFirstVideo(page);
+
+    await page.getByTestId("player-close").click();
+    await expect(page.getByTestId("player-modal")).toHaveCount(0);
+  });
+
+  test("closes on Escape", async ({ page }) => {
+    await openApp(page);
+    await openFirstVideo(page);
+
+    await page.keyboard.press("Escape");
+    await expect(page.getByTestId("player-modal")).toHaveCount(0);
+  });
+});