fix(frontend): lock page scroll on iOS and pin top-bar for iPad

fix(frontend): lock page scroll on iOS and pin top-bar for iPad

- Use body{position:fixed; inset:0; overflow:hidden} to reliably lock the
  page on iOS Safari (overflow:hidden on body is ignored by iOS).
- Remove ineffective position:sticky from .top-bar; with body fixed the
  top-bar is a flex-shrink:0 flex item pinned at the top naturally.
- Fixes two iPad issues: title bar scrolling away, and page not
  auto-scrolling to newest terminal output (xterm handles internal scroll).