Install prompt updated.

Changes:
- It no longer appears immediately when the homepage loads.
- It appears after the user scrolls about 220px OR after about 12 seconds.
- It is positioned at the bottom, so it does not cover the logo/header/menu.
- It works on all pages, not only the homepage.
- Android Chrome uses the native PWA install prompt.
- iPhone/iPad uses a custom premium instruction banner: Share -> Add to Home Screen.
- Close behavior:
  1st/2nd close: may show again later after about 90 seconds.
  3rd close: pauses for 6 hours.
- Installed/standalone mode: no banner.
