[e.cash] Fix mobile video display issue

Summary:
Some users are seeing the background video jump to full screen on mobile causing bad UX and sometimes a blank screen effect.
For example on /build, /roadmap, /get-ecash
It seems to be unique to iOS safari which has an autoplay fullscreen feature in it apparently
Even setting the video to display none the issue persisted, so opting to conditionally render the video based on screen size and show a still frame on mobile
The video playback speed is a relic of the stealthmode theme, and the ref was causing some issues here when conditionally rendering so also taking that out here

Test Plan: Preview the site and check some subpages out to see if any display issues happen on mobile

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16110