fix #202

Popup width is fixed to 320px when the extension
is pinned to overflow menu. The fixed width might
change in the future, but the popup is responsive
so we're only setting a max-width rather than a
fixed width.

pages/popup/popup.css

@@ -11,7 +11,7 @@ body {
     margin: 0 auto;
     overflow: hidden;
     padding: 0;
-    width: 348px;
+    max-width: 348px;
 }
 
 header {