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/helpers.ts

@@ -0,0 +1,20 @@
+import { expect, type Page } from "@playwright/test";
+
+/** Open the app and wait for the authenticated shell (the nav rail) to be ready. */
+export async function openApp(page: Page, path = "/"): Promise<void> {
+  await page.goto(path);
+  await expect(page.getByTestId("nav-feed")).toBeVisible();
+}
+
+/** Click a top-level module in the nav rail and confirm it becomes the active page. */
+export async function gotoPage(page: Page, id: string): Promise<void> {
+  await page.getByTestId(`nav-${id}`).click();
+  await expect(page.getByTestId(`nav-${id}`)).toHaveAttribute("aria-current", "page");
+}
+
+/** Open the in-app player from the first feed card (clicks its thumbnail link). */
+export async function openFirstVideo(page: Page): Promise<void> {
+  await expect(page.getByTestId("video-card").first()).toBeVisible();
+  await page.getByTestId("video-card").first().getByRole("link").first().click();
+  await expect(page.getByTestId("player-modal")).toBeVisible();
+}