[FIX] im_livechat: make bubble background behind message content

Before this commit, bubble background was on top of message content.
This prevent user interaction on the message.
This commit fixes the issue by properly putting background behind text.

closes odoo/odoo#99020

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

addons/im_livechat/static/src/legacy/public_livechat.scss

@@ -687,6 +687,7 @@ $o-mail-thread-window-zindex: $zindex-modal + 1 !default;
 
 .o_PublicLivechatMessage_background {
     position: absolute;
+    z-index: -1;
     left: 0;
     top: 0;
     width: 100%;