[FIX] point_of_sale: background layout header iframe

When a report with header is displayed in iframe with layout
"Background", part of the header maybe outside of the page.

For example on /my/invoices portal show of an invoice, the max width
report iframe can set at 210mm (+/- 793.7px normally), the header in
background layout has `min-width: 900px` so is wrongly centered and may
have content outside of shown page (eg. company tagline).

The same issue happen when using HTML rendering of the report in
backend, but depends on the viewport width.

This may be the expected configuration when printing (wkhtmltopdf should
unzoom if something was too large to fit) but not when displaying html
directly.

opw-1915807
closes #32373

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/web/static/src/scss/layout_background.scss

@@ -82,3 +82,10 @@
         }
     }
 }
+
+/* special case for displaying report in iframe */
+.o_in_iframe {
+    .o_background_header {
+        min-width: 0;
+    }
+}