feat: Display received time and fix timestamp timezone conversion.

frontend/src/views/Home.vue

@@ -64,6 +64,7 @@
                 <h3>{{ selectedMessage.subject }}</h3>
                 <p class="from-line"><strong>{{ $t('home.from') }}:</strong> {{ selectedMessage.sender }}</p>
                 <p><strong>{{ $t('home.to') }}:</strong> {{ selectedMessage.recipient }}</p>
+                <p><strong>{{ $t('home.received_at') }}:</strong> {{ selectedMessage.received_at }}</p>
               </div>
               <div class="header-actions">
                 <button @click="deleteMessage(selectedMessage.id)" :title="$t('home.delete')">