💄(frontend) max img width with container width

The img could be much bigger than the page width
in a generated pdf. We limit the max img width
to the container width.

src/backend/demo/data/template/css.txt

@@ -13,3 +13,6 @@ body {
 .body{
   margin-top: 1.5rem
 }
+img {
+  max-width: 100%;
+}