fix: add max-width to panel in landscape mode

In some situation in mobile (eg. when the keyboard is used), the
screen is still considered as landscape, but it is very small, so
make sure the panel does not overflow on the right.

cf #1788

umap/static/umap/css/panel.css

@@ -90,6 +90,7 @@
         top: 0;
         margin-top: var(--panel-gutter);
         width: var(--panel-width);
+        max-width: calc(100% - var(--panel-gutter) * 2 - var(--control-size))
     }
     .panel.condensed {
         max-height: 500px;