[FIX] mail: sets the z-index of dialog to the z-index of modals

This commit gives to o_Dialog the same z-index as modals, before
this commit, the dialog could be hidden or partially obstructed
by other elements.

part of task-2171884

closes odoo/odoo#60162

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

addons/mail/static/src/components/dialog/dialog.scss

@@ -11,7 +11,7 @@
     display: flex;
     align-items: center;
     justify-content: center;
-    z-index: 1;
+    z-index: $zindex-modal;
 }
 
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