I connect Quasar to Nuxt as a plugin - icons are not displayed in QBtn, what should I do?

Question

Vue.js

I connect like this:
plugins/quasar.js

import Vue from 'vue';
import "quasar/dist/quasar.css";

import Quasar, { QBtn } from "quasar";
Vue.use(Quasar, { components: [QBtn] });

pages/index.vue

<template>
  <div class="container">
    <q-btn color="primary" icon="mail" label="On Left" />
  </div>
</template>

I have already tried everything with icons, from here: https://quasar.dev/options/quasar-icon-sets#Vue-CLI-Way and from here: https://quasar.dev/options/quasar-icon-sets# Quasar ... and also poked for two hours - to no avail.
Tell me what's wrong please.
//specially put a clean install of Quasar - the icons there work without any settings at all...