bug(web): blank space at bottom of page in mobile use is visible when keyboard is not active

This is arising from https://community.software.sil.org/t/keman-keyboard-is-not-rendering-on-chorme-and-mozilla-browsers/9783/15.

The site developer noted an issue where a large white space was being added to the bottom of their page, below what was intended as the page's footer. This doesn't fit their design and led them to ask about it. It's actually a longstanding behavior of KMW:

https://github.com/keymanapp/keyman/blob/dd17fa50c57b33fc572a57f04b7a31f2fbd1c4b4/web/src/app/browser/src/context/pageIntegrationHandlers.ts#L56-L63

Note that this trailer:
- is not sized to match the height of the OSK
- does not hide when the OSK is not displayed

This is not ideal for optimal integration of KMW with other sites, especially when they have non-matching formatting at the bottom of the page. That said, it _is_ longstanding behavior of Keyman Engine for Web.