Add Playwright E2E tests replacing Cypress with MSW integration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/e2e/specs/notFound404.spec.ts

@@ -0,0 +1,44 @@
+// The Lucia project.
+// Copyright 2026-2026 DSP, inc.  All rights reserved.
+// Authors:
+//   imacat.yang@dsp.im (imacat), 2026/03/22
+
+import { test, expect } from "@playwright/test";
+import { loginWithMSW } from "../helpers";
+
+test.describe("404 Not Found Page", () => {
+  test("displays 404 page for non-existent route", async ({
+    page,
+    context,
+  }) => {
+    await loginWithMSW(context);
+    await page.goto("/this-page-does-not-exist");
+    await expect(page.getByText("404")).toBeVisible();
+    await expect(
+      page.getByText("The page you are looking for does not exist."),
+    ).toBeVisible();
+  });
+
+  test("has a link back to Files page", async ({
+    page,
+    context,
+  }) => {
+    await loginWithMSW(context);
+    await page.goto("/some/random/path");
+    const link = page.getByRole("link", { name: "Go to Files" });
+    await expect(link).toBeVisible();
+    await expect(link).toHaveAttribute("href", "/files");
+  });
+
+  test("displays 404 for unauthenticated user on invalid route", async ({
+    page,
+  }) => {
+    await page.goto("/not-a-real-page");
+    const url = page.url();
+    if (url.includes("/login")) {
+      await expect(page).toHaveURL(/\/login/);
+    } else {
+      await expect(page.getByText("404")).toBeVisible();
+    }
+  });
+});