fix 404 page for mobile

src/pages/ErrorNotFound.vue

 <template>
   <div class="fullscreen bg-secondary text-white text-center q-pa-md flex flex-center">
     <div>
-      <div style="font-size: 40vh" class="text-accent">
+      <div :style="$q.screen.lt.sm ? 'font-size: 30vh' : 'font-size: 40vh'" class="text-accent">
         404
       </div>
 
-      <div class="text-h3">
+      <div :class="$q.screen.lt.sm ? 'text-h4' : 'text-h3'">
         Le saviez-vous ?
       </div>
 
-      <div class="text-h4">
+      <div :class="$q.screen.lt.sm ? 'text-h5' : 'text-h4'">
         Le code 404 signifie juste "page introuvable" ! 🤯
       </div>