[FIX] snailmail: background layout footer

In the background layout footer, it happens that the content is over a
zone that must stay blank. That's why w've added a left-padding to the
footer.

OPW-1928366

closes odoo/odoo#30296

addons/snailmail/static/src/scss/snailmail_external_layout_asset.scss

@@ -14,6 +14,9 @@
         padding-left: 10mm;
     }
 }
+.o_background_footer {
+    padding-left: 8mm;
+}
 .article>.page {
     > h1, > h2, > h3 {
         width: 50%;