[FIX] mail: fix dropzone background for live chat

Before this PR, the dropzone background was not showing on
the embed live chat. The dropzone is displayed as a `main
component`. In the Odoo frontend, the main component container
is displayed outside of the live chat shadow dom, where the
`bg-view` class is not available. This commit replaces this
class by the `bg-100` one that is almost the same and available
in the frontend.

task-3595537

closes odoo/odoo#142055

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/core/common/dropzone.xml

@@ -2,7 +2,7 @@
 <templates xml:space="preserve">
 
 <t t-name="mail.Dropzone">
-    <div class="o-mail-Dropzone position-fixed align-items-center justify-content-center d-flex border-primary bg-view text-primary opacity-75"
+    <div class="o-mail-Dropzone position-fixed align-items-center justify-content-center d-flex border-primary bg-100 text-primary opacity-75"
         t-att-class="{ 'o-dragging-inside': state.isDraggingInside }"
         t-attf-class="{{ props.extraClass }}"
         t-on-dragenter="() => state.isDraggingInside = true"