[FIX] web_editor: hide inactive modal behind fullscreen editor

`.modal.o_inactive_modal` (in modal.scss) had a z-index that was
superior to that of a fullscreen editor. As a result we could end up
with the editor being partially hidden by a modal.

task-2741872

closes odoo/odoo#85315

X-original-commit: 6ef3c1cf59ac66f00462468079500c449f189fae
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/scss/web_editor.backend.scss

@@ -56,7 +56,7 @@
         bottom: 0 !important;
         width: 100% !important;
         min-height: 100% !important;
-        z-index: 1001 !important;
+        z-index: $zindex-modal-backdrop + 1 !important;
         border: 0;
     }
     .tooltip-field-info {