feat(auth): unify mobile-first authentication flow

src/pages/auth/LoginOtpPage.tsx

@@ -37,7 +37,7 @@ export function LoginOtpPage() {
   const [now, setNow] = useState(Date.now())
 
   if (!state.login.mobile) {
-    return <Navigate to="/auth/login" replace />
+    return <Navigate to="/auth" replace />
   }
 
   useEffect(() => {
@@ -181,6 +181,9 @@ export function LoginOtpPage() {
     <AuthPanel
       title={t.login.loginOtpTitle}
       description={t.login.sentCodeDesc(state.login.mobile)}
+      backTo="/auth/login/password"
+      backLabel={t.login.backToPasswordLogin}
+      step={{ current: 2, total: 2, label: t.login.stepLabel(2, 2) }}
       alert={throttleAlert}
     >
       <form