Add Discover tab navigation and 404 page E2E tests

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

cypress/e2e/notFound404.cy.js

@@ -0,0 +1,34 @@
+import { loginWithFixtures } from '../support/intercept';
+
+describe('404 Not Found Page', () => {
+  beforeEach(() => {
+    // Suppress Navbar error on 404 page (route.matched[1] is null)
+    cy.on('uncaught:exception', () => false);
+  });
+
+  it('displays 404 page for non-existent route', () => {
+    loginWithFixtures();
+    cy.visit('/this-page-does-not-exist');
+    cy.contains('404').should('be.visible');
+    cy.contains('您想找的頁面不存在').should('be.visible');
+  });
+
+  it('has a link back to Files page', () => {
+    loginWithFixtures();
+    cy.visit('/some/random/path');
+    cy.contains('a', '回到 Files 頁')
+      .should('be.visible')
+      .should('have.attr', 'href', '/files');
+  });
+
+  it('displays 404 for unauthenticated user on invalid route', () => {
+    cy.visit('/not-a-real-page');
+    cy.url().then((url) => {
+      if (url.includes('/login')) {
+        cy.url().should('include', '/login');
+      } else {
+        cy.contains('404').should('be.visible');
+      }
+    });
+  });
+});