Scroll height is too large

[stevengharris]

Since implementing the fix for #106 by adding a padding-block of 100% in markup.css, the scroll height when focused has been too large on Mac Catalyst. The 100% value makes the padding-block height the viewport height, which is the device height. This is generally fine for a phone, but when used with Mac Catalyst, the scroll height prevents proper auto-sizing of the WKWebView height based on its contents. The fix for this issue is to explicitly set the padding-block bottom based on the content height and the WKWebView frame height, so it fills the full height, not 100% of the viewport.