[FIX] mail: better separate header and content of chat window visually

Before this commit, content inside chat window could visually fuse
with the header in a way that makes it hard to see the separation
between the header and the content.

This commit fixes the issue by adding a small border below header
when there is some content.

closes odoo/odoo#119173

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/web/chat_window/chat_window.xml

@@ -13,7 +13,7 @@
         t-on-keydown.stop="onKeydown"
         tabindex="1"
     >
-        <div class="o-mail-ChatWindow-header d-flex align-items-center flex-shrink-0" t-on-click="toggleFold" t-att-class="{ 'cursor-pointer': !store.isSmall }">
+        <div class="o-mail-ChatWindow-header d-flex align-items-center flex-shrink-0" t-on-click="toggleFold" t-att-class="{ 'cursor-pointer': !store.isSmall, 'border-bottom': !props.chatWindow.folded }">
             <div t-if="store.isSmall" class="cursor-pointer d-flex align-items-center h-100 px-3 py-0 me-2" title="Close conversation" t-on-click="() => this.close()">
                 <i class="fa fa-arrow-left"/>
             </div>