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/playwright.config.ts

@@ -0,0 +1,51 @@
+import { defineConfig, devices } from "@playwright/test";
+
+/**
+ * E2E characterization (golden-master) net for the Platform refactor.
+ *
+ * Target (PW_BASE_URL): defaults to the baked localdev stack on :8080 (production-faithful,
+ * same-origin, no HMR/WS gaps). Point it at http://localhost:5173 for the fast Vite loop — all
+ * five characterization flows are proxy-safe (none use the WebSocket).
+ *
+ * The stack must already be UP (owned by `siftlode dev`); there is deliberately no `webServer`.
+ * `e2e/global.setup.ts` seeds the dedicated test account and logs in once, sharing the resulting
+ * session via storageState so the real tests never re-authenticate (auth endpoints are rate-limited).
+ *
+ * Runs serial / single-worker on purpose: the specs share ONE seeded account, and a couple of
+ * them mutate server state (Settings save), so determinism beats parallelism here.
+ */
+
+const BASE_URL = process.env.PW_BASE_URL || "http://127.0.0.1:8080";
+const STORAGE_STATE = "e2e/.auth/state.json";
+
+export default defineConfig({
+  testDir: "./e2e",
+  fullyParallel: false,
+  workers: 1,
+  forbidOnly: !!process.env.CI,
+  retries: 0,
+  reporter: [["list"], ["html", { open: "never" }]],
+  timeout: 30_000,
+  expect: { timeout: 7_000 },
+  use: {
+    baseURL: BASE_URL,
+    storageState: STORAGE_STATE,
+    locale: "en-US",
+    // retain-on-failure (not on-first-retry): with retries:0 a "first retry" never happens, so a
+    // trace would never be captured — this keeps a full trace for any failure, discarded on success.
+    trace: "retain-on-failure",
+    screenshot: "only-on-failure",
+  },
+  projects: [
+    {
+      name: "setup",
+      testMatch: /global\.setup\.ts/,
+      use: { storageState: { cookies: [], origins: [] } },
+    },
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"], storageState: STORAGE_STATE },
+      dependencies: ["setup"],
+    },
+  ],
+});