website: more e2e tests for signin

Fixed issue where existing sign in test fails due to the existence of dev test login.

Added reusable cy.signInWithEmail() to login before testing rest of UI.

website/src/components/Header/UserMenu.tsx

@@ -34,7 +34,9 @@ export function UserMenu() {
                   height="40"
                   className="rounded-full"
                 ></Image>
-                <p className="hidden lg:flex">{session.user.name || session.user.email}</p>
+                <p data-cy="username" className="hidden lg:flex">
+                  {session.user.name || session.user.email}
+                </p>
               </div>
             </Popover.Button>
             <AnimatePresence initial={false}>